More details are emerging about Megan Fox and Machine Gun Kelly's tumultuous relationship.

As rift rumors swirl around the engaged pair, a source told ET that they "got into a huge argument before their performance at

the Sports Illustrated event on Saturday night." ET has reached out to representatives of Fox and MGK for comment.

The source says, "Megan and MGK have a passionate relationship full of strong emotions, both good and bad,

but Megan was very upset after this fight." "She was initially scheduled to appear on the program with MGK and had a full outfit planned,

but she backed out at the last minute."  According to the source, the apparent argument affected MGK's performance.

The source adds, “MGK was not in the right frame of mind to perform after this.”

"He wasn't himself and his band carried the show on their backs. It wasn't a typical MGK performance or what people expected of them.

While performing 'Bloody Valentine' their mic died and they taped

it and someone seemed pissed off before coming in to change that."Usually after a show,