While Kelsea Ballerini and Morgan Evans' divorce resulted in "he said, she said," 

Page Six can exclusively reveal what allegedly happened before the former couple's marriage fell apart.  

An insider tells us the country-pop singer cheated on her ex-husband with

Chainsmokers member Drew Taggart at Lollapalooza in August 2019 — after collaborating on their hit 2018 track, 

"This Feeling" — but Will not come clean to Evans about it until 2021. 

We're told the alleged affair, coupled with other issues, 

led Ballerini, 29, to call it quits with Evans, 37, last summer and "blindsided" him with her decision.

Ballerini's rep did not return Page Six's requests for comment. 

Reps for both Taggart and Evans had no comment. 

A source close to Taggart, who has recently been linked to Selena Gomez, 

told Page Six that the DJ, 33, was single at the time of his meeting with Ballerini.