Actress Serayah McNeill's decision to slam her ex-fiancé Joey Bada$$ over alleged infidelity was sparked by a phone call she heard just last week, TMZ has learned. 

Sources close to the situation tell TMZ ... Serayah heard the call between Joey and another woman, where the woman was talking trash, saying Serayah was embarrassed her engagement was ending and was simply lashing out at Joey.

Sources tell TMZ Serayah believes Joey and the woman were messing around behind her back … which led to her posting a video slamming the rapper earlier today, which she later deleted.

On Monday morning, Serayah also posted and deleted a snippet of Joey's convo with the woman. 

As TMZ previously reported, Serayah said she made the decision to end her engagement with Joey, with whom she shares a kid. 

She accused him of repeated infidelity and said he needs to be “shown for exactly who he is.” 

We've reached out to Joey for comment, but no word back yet.