Rapper Diddy believes his ex-girlfriend Cassie Ventura cheated on him with the personal trainer he hired to work with her, a source reveals to PEOPLE

Sean “Diddy” Combs’ ex-girlfriend Cassie Ventura just went Instagram official with professional bull rider Alex Fine, but the relationship may not be as new as it appears — at least Diddy doesn’t think so.

The music mogul, 49, believes Ventura, 32, cheated on him with Fine, a source tells PEOPLE. Ventura just introduced her new beau to her followers on Saturday when she posted a photo of herself and Fine sharing a kiss on the lips.

But Fine and Ventura have known each other for months, the source says. The pair met when Diddy hired the bull rider to be her personal trainer before their split was announced in October, the insider claims.

The source explained why Diddy believed “there was absolutely overlap.” Said the source: “Cassie wanted a personal trainer, so Diddy hired Alex for her. Then Cassie and Alex started hooking up before her relationship with Diddy was over.”

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Cassie and Diddy
| Credit: Neilson Barnard/Getty

Ventura’s Instagram post of Fine, which also included a shot of her mother from a Christmas party, came just one day after Diddy made his affection for his ex-girlfriend known.

On Friday, Diddy shared a smiling photo of the singer, in which she appeared to be taking a bubble bath.

Adding a heart emoji over the top of the image, Diddy also tagged the singer in the photo, although Ventura no longer follows him on Instagram.

Representatives for the former couple and Fine did not immediately respond to PEOPLE’s request for comment.

Their split came after 11 years of dating. A different source told PEOPLE their “decision was amicable and they remain friends,” adding that Ventura plans to “focus on her music and acting career.”

Up until now, the pair appeared to be on pretty good terms.

One day after Diddy’s ex Kim Porter’s death in November, the rapper was seen reuniting with Ventura in Los Angeles.

Cassie and Diddy
| Credit: Kevin Mazur/Getty Images

The singer went on to share her own heartfelt memorial for Porter on social media, during which she also praised Diddy for being a wonderful father to his six children.

“There are no words….An amazing mother to her beautiful family, lit up every room she entered and now the most beautiful angel,” Ventura wrote, alongside a glamorous photo of Porter.

On Thursday, Diddy revealed he’s still coping with the loss of Porter, explaining that his family has helped him through depression.

“GOD IS SO GOOD. THE GREATEST!!! I’m the luckiest man in the world. The way my family has been strong has humbled me,” Diddy tweeted a shot of his mother Janice cuddling up to his twins Jessie and D’Lila with Porter.

“They have led me. They brought me out of a state of deep depression. Their strength and love brought me back into the light. I love my family!!”