During The Women Tell All special that aired Tuesday night on ABC, Cat yelled out, "Anastasia, you had a boyfriend on the show! You called him in The Bahamas when you got your phone back!"
Zach said of the startling accusation, "I don't know if it's true or not. But that got me. That was a curveball."
Anastasia -- a content marketing manager who had already been under fire for allegedly going on the show for the wrong reasons, to gain social-media followers -- appeared stunned by the allegation.
Cat claimed that Anastasia was in a relationship with the man who had agreed to watch her dog while she filmed The Bachelor.
Anastasia denied the allegation and insisted her mother had watched her dog while she traveled for the show. And her pal Genevie Mayo said that as Anastasia's roommate, she would've known if Anastasia had a secret boyfriend back home.
"I have receipts!" Cat alleged. "Don't lie!"
But Anastasia told Jesse, "I didn't have a boyfriend, I still don't have a boyfriend, and I never dated that person [on my Instagram]."
Anastasia said the photos of the mystery man had been taken at a friend's wedding in Seattle one week before the show filmed and they were fun and innocent pictures.
"Okay keep lying Anastasia and maybe you'll hit the 50k [followers] you wanted so bad," Victoria Jameson snapped.
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Zach faced a room full of his brokenhearted and jilted exes on The Women Tell All. While none of the bachelorettes were angry at him, Jess Girod and Katherine "Kat" Izzo wanted answers.
Both women had been blindsided by their eliminations and sought clarity on why Zach could no longer see a future with them.
"What was important to me was the women having closure on the relationships," Zach said of the experience.
"It's tough because I'm making these decisions with my heart and gut instinct. It becomes clear to me and then I have to move on to the next week."
Zach apparently found the tense and emotional conversations to be cathartic.
"It was really important for me to talk to them and let them know what I was feeling," Zach shared. "Like, 'No, I really did like you so much.' It was really nice and helpful for me. And hopefully for the women."
The Bachelor's 27th season is winding down with Zach's Fantasy Suite dates airing next week on ABC.