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.
Cassie sure seems smitten with Alex. She has not ever publicly talked about a timeline, but we're told Diddy is convinced there's an overlap.
Sources connected to Cassie insist she did NOT start dating Alex until after she broke up with Diddy.
We reached out to Alex ... so far, no word back.