A source close to Christina told In Touch Weekly that Tarek "cannot let go" of his Flip or Flop co-star and is "putting her through hell," the Daily Mail reported.
Although Christina, 33, seems to be happily dating hew new beau, Doug Spedding, 55, it allegedly bothers her how Tarek is bragging in the press about how much fun he's having dating multiple women at once and living the bachelor lifestyle as a "free spirit."
"It's like [Tarek's] going out of his way to push her buttons. And it's getting to her. All she wants to do is move on. It's outrageous," the source reportedly said. "If she did that, Tarek would be livid."
Christina and Doug have been dating for about a month now, and she's all smiles when they're photographed on date nights and holding hands in public. She recently reconnected with Doug, a man she has known for 10 years whom she dated before ever getting romantically involved with her ex, Tarek.
"Christina puts up a good front, but she's broken down in tears over the stress of dealing with Tarek and is trying to keep it all together," the source told In Touch. "She tries to be strong, but behind the smiles, there's heartache and tears."
It's been about one year since Tarek and Christina quietly split after seven years of marriage and began dating other people, but the HGTV reality stars -- who have two children together, Taylor, 6, and Brayden, 20 months -- didn't publicly announce their decision to part ways until December 2016.
Tarek officially filed for divorce from the blonde beauty in January 2017.
"[Tarek] still checks on her through family and mutual friends," the source said, according to the Daily Mail.
"Tarek's also obsessed with her Instagram, checks it regularly, sees who she follows and who follows her. And if he's not happy about something he sees or that she posts, he will immediately let her know about it. It's a lot to deal with."
Despite alleged tension between the couple, they are still filming episodes of Flip or Flop to premiere in December 2017. HGTV had ordered 20 new episodes, which is more than a traditional full-season order.
Prior to her romance with Doug, Christina appeared to have a brief fling with one of the family's former contractors, Gary Anderson, as well as NHL player Nate Thompson.
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Tarek, 35, has been linked to one of the couple's former nannies, Alyssa Logan, as well as several unidentified women.