"Life is short. Do what makes you happy," Katie, 30, captioned the sweet photo, along with a heart emoji.
Comments poured in from her followers, and many Instagram users expressed skepticism and disapproval considering Katie and Blake had just announced their decision to call off their engagement and part ways in late October.
Katie was called "desperate" and haters said she "didn't waste any time" moving on from Blake, but Katie clarified in the comments how she and Blake actually got engaged in April, although their engagement didn't air on The Bachelorette's seventeenth season until August.
Someone also called John her "rebound," and another person mentioned how Katie "will be with someone else come New Years."
One troll wrote to Katie, "Genuine question? Does that mean when you are bored of him you will get another because [of] 'happiness?'"
The former bank marketing manager from Washington boldly replied, "Being 'bored' and being 'unhappy' are very different."
"If something doesn't bring joy into my life (friendships, relationships, a job, etc.) -- yes, I'm going to remove that from my life."
Katie previously hinted on social media that she and Blake were fighting too much during their six-month relationship and broke up due to their different communication styles.
The TikTok star and social-media influencer suggested that Blake was unable to make her happy and vice versa.
Blake also said on the "Talking It Out with Bachelor Nation" podcast that he and Katie have different love languages and their similarities, such as being outgoing and stubborn, worked against them instead of for them when it came to romance.
Katie, however, also received her fair share of supportive and encouraging comments on her Friday photo with John.
One person told Katie that she had been "yelling at the TV" for Katie to choose John on The Bachelorette earlier this year.
But Katie replied, "It would have never worked for us on the show. He was too shy. A solid friendship first built an amazing foundation."
Katie also took to her Instagram Stories on Sunday to answer more questions about her romance with John.
When asked if she fell in love with John before or after her engagement to Blake was called off, Katie responded, "Always loved him as my best friend."
"Then one day I looked at him and saw him differently. Like Cupid to the heart, and I knew I was hit," Katie added, somewhat dodging the question.
Katie also confirmed she's still in "the honeymoon stage" of her relationship.
Katie and John just made their red carpet debut together on Thursday at their mutual friend Stephen Lovegrove's Christmas party.
Katie and John, whom she eliminated in Week 2 of The Bachelorette's seventeenth season, went Instagram official with their relationship on November 23, just three weeks after Katie and Blake announced their breakup.
Katie's ability to move on so quickly from her fiance raised eyebrows, and Blake said on the "Talking It Out with Bachelor Nation" podcast in late November, "I don't think there was any physical cheating there, but there's clearly emotional [cheating] for it to transition as quickly as it did [from friendship to romance]."
John therefore took to Reddit days later with a lengthy statement to set the record straight on the circumstances in which he and Katie began dating.
"I'm sure there's no question why I've chosen this moment to speak up -- this has been a wild couple days to say the least," John wrote.
"What hurts me... is watching SO many of you build assumptions and throw around accusations based on very limited knowledge of a situation."
He went on to explain, "Katie and I developed an incredible and deep friendship over the last half year or so. The circumstances by which this friendship formed was a series of crazy, random, and coincidental events, that I'm sure you will all hear about at a later time."
"My point for now, is that we were ALWAYS platonic," he clarified. "Even during my short stint on the season, we never had a chance to foster anything romantic (read as: John blew his chance and left the next day hahaha)."
John suggested that he totally respected Katie and Blake's relationship while they were together for six months.
"The boundaries of our friendship with respect to her ongoing relationship never even required addressing, because it was never a thought in our minds that we would ever be anything more. You guys -- she was engaged," John explained.
"It is apparent that many of you misjudge how both her and I view that commitment. Neither Katie nor I would ever cheat, push someone to cheat, or be involved in cheating."
John insisted it's not in their "nature" to cheat on someone and such behavior "is not acceptable."
"It is not okay. On second thought, maybe this message deserves its own paragraph: There was no wrongdoing prior to the end of their engagement," John wrote.
"Furthermore, I understand that the timeline seems fast and messy. But I urge you to put yourselves in our shoes. Once Katie and I realized there was a romantic connection that we were interested in exploring, we were not starting from scratch; we were building off of the foundation of a close friendship through which, we had already gotten to know each other so well."
John apparently fell for Katie hard and fast once romance became an option for them.
"The speed at which I fell for this incredible, nurturing, and genuinely caring human being was honestly terrifying. I was scared. But every step of the way felt so right," John shared.
"We knew that the only difficult part would be the optics of it all. And that's a sh-tty thing -- worrying about how a natural and organic connection will be viewed by hundreds of thousands of people... when was the last time you had to think about that when you fell for a partner?"
John clarified the rapid rate at which he and Katie became a serious couple was "sure as hell" a first-time experience for him.
John also recognizes that many The Bachelor fans have disagreed and will still disapprove with how he and Katie went about their new relationship.
"To those of you that suggest we should have waited -- I understand. I hear you," John noted.
"But try to understand what it's like for me, a regular guy, falling helplessly for his best friend, not being able to take her out for dinner because photos would likely leak and everybody would assume we were trying to foster some sort of secret and scandalous relationship haha."
He elaborated, "Our decision to come out publicly was a decision to pursue happiness in the way we wanted. Disagreeing with someone for how they decide to go about it is okay. Everybody is different. I understand."
John said the idea his relationship with Katie was a source of anxiety for anyone "deeply saddens" him.
"As I mentioned before, cheating in any form is unacceptable, and I'm sorry if it looked as though either of us were taking advantage of anyone's trust," John continued.
"I have tried to respond directly to those individuals that were affected, but I know that I don't have the capability of finding absolutely everyone; so I hope this message finds you well."
Katie went public with her new romance last week by posting footage of John on her Instagram Story set to Taylor Swift's "Begin Again," a song about falling in love all over again.
The John dedication served as the conclusion of Katie's "12 Days of Messy" challenge in which she had been tasked with matching every song on Taylor's re-released "Red" album to one of her Season 17 bachelors.
Blake called Katie's romance "a bomb drop" on him. Blake told Mike and Bryan that he feels "stupid and foolish for allowing" Katie and John to reconnect in such an intimate way.
"I don't know if it was partially me too. Like, was she in some sense emotionally cheating to some degree because I wasn't able to provide something with words of affirmation?" questioned Blake.
The wildlife manager from Canada acknowledged how he stands by "having no regrets" in pursuing and getting engaged to Katie but the way things are playing out is making that more challenging.
Speculation Katie and John are dating began swirling in the summer, when Katie posted footage of herself at a bar John works at in San Diego, which prompted rumors John may have been the winner of her The Bachelorette season.
Katie proceeded to move to San Diego and continue hanging out with John while Blake traveled to Africa to pursue his wildlife conservation efforts.
Katie and Blake attempted to make a long-distance relationship work as people simultaneously questioned if sparks were flying between Katie and John.
"Internet rumors lead me to this guy becoming a dear dear friend," Katie reportedly wrote on Instagram in September after enjoying a day out with John in California.
While Katie was trying to mend her broken heart in late October, she uploaded a photo of John's dog to her Instagram Story and wrote, "Dexter forcing me out of the house this morning."
Katie had addressed the John dating rumors in early November but danced around an official confirmation or denial.
At the time, Katie shared a throwback video of "the first time I saw John since filming" on her Instagram Story that showed Polaroid photos of the supposed close pals playing cards together, Us reported.
"Y'all getting wild in my DMs," Katie wrote along with a laughing emoticon on November 7.
Blake said on "Talking It Out" that although his breakup with Katie was "mutual," her feelings for John may have helped their engagement to "abruptly end the way it did and as easy as it did."
"Even though it was mutual, was he always in her back pocket?" Blake wondered. "I don't know."
Blake therefore suggested John may have been Katie's "Plan B" based on the timeline.
Blake said John must have been providing Katie with something he wasn't giving her -- but he is now "done with the messiness."
Another reason the pair may have broken up? An insider told The Sun that Katie's "outbursts eventually became too much" for Blake "to handle" during their relationship.