Cassie dumped longtime boyfriend Diddy for her trainer - now Diddy is claiming that Cassie "betrayed" the mogul, by cheating on him.

Diddy has been known to openly cheat on Cassie during the course of their 10 year relationship. In fact, many suspect that Diddy slept with as many as 100 different women while dating Cassie.

This year, Cassie finally had enough and left Diddy - for her personal trainer Alex Fine.

Sources connected to Cassie though have insisted she didn't start dating Alex until after she broke up with Diddy.

Today Diddy's reps leaked a story to TMZ - claiming that he's upset that Cassie cheated on him. Here's what is being said:

 Sources claim the guy he hired to keep Cassie in shape was banging her on his dime ... 

Here's what we're told. Diddy's trainer, Jamal Liggin, got so busy he hired several trainers as backup, including Alex Fine. Diddy tagged Alex to train Cassie over the last year.

As we reported, Diddy and Cassie broke up several months ago after a decade-long relationship, but our sources claim Cassie and Alex hooked up before the split. We're told what especially galls Diddy ... he paid Alex to train his then-girlfriend and feels betrayed. 

On Wednesday Cassie shared an Instagram post kissing a new mystery man - revealed to be 25-year-old celebrity personal trainer Alex, on Saturday.

The beauty was makeup free as she snuggled next to the professional bull rider while on a boat.

'Baby, she captioned the photo that was taken somewhere in the Pacific Ocean. Alex also shared the same selfie style shot with the caption: ''19.'