Kristen Stewart BUSTED cheating on Robert Pattinson with someone from the Snow White movie! (PHOTO)

Kristen Stewart and Robert Pattinson finally admitted last year that they were dating and have been on a public love fest ever since. Everything seemed to be going good for them in their Twilight love land until Kristen was busted cheating with someone from another one of her movies.

Last week, Kristen (22) was photographed making out with her ‘Snow White and the Huntsman’ director, 41-year old Rupert Sanders. Rupert is married to this British Vogue model and has two young kids.

A source said that this wasn’t an affair, just a ‘fleeting moment that never should have happened’. Kristen is devastated because it was a mistake that never should have happened and she never meant to hurt anyone.

Rob does know about Kristen’s wandering mouth but it’s not clear when he found out. He and Kristen just attended the ‘Teen Choice Awards’ together over the weekend.

US Weekly magazine is the one that busted them and will have more on this story in this weeks issue. Check out the cover of this week’s issue below that shows one of the pictures of Kristen and Rupert canoodling.