Kristen Stewart Cheats On Robert Pattinson

The relationship between Kristen Stewart and Robert Pattinson has been rocked by the revelation that the actress had a brief fling with a married director, sources tell WHO Magazine.

"Kristen is absolutely devastated," says one source about the brief romance between the actress and her Snow White & the Hunstman director, Rupert Sanders. "It was a mistake and a complete lapse in judgment."

The hookup with Stewart's Snow White and the Huntsman director Rupert Sanders occurred recently, the source says.

"She wasn't having an affair with Rupert. It was just a fleeting moment that shouldn't have happened," says the source. "She never meant to hurt anyone. She's a good person who just made a bad choice."

A rep for Sanders, who is married and has two children, could not be reached for comment. Sanders wife, Liberty Ross, also played Stewart's on screen mum in the summer blockbuster, Snow White & The Hunstman.

For more on Kristen's affair, be sure to check WHO.com as the story unfolds.

PHOTOS: Kristen & Charlize's Sheer Streak