Kristen Stewart allegedly cheated on longtime boyfriend Robert Pattinson.

According to Us Weekly’s new issue, the cover of which leaked online Tuesday afternoon, Pattinson made a "heartbreaking discovery" when he found out Stewart allegedly had a "steamy affair" with her "Snow White" and the "Huntsman" director Rupert Sanders.

It has to be noted that a small photo featured on the cover seemingly shows Sanders, who is married with children, wrapping his arms around Stewart in an embrace.

Us further claims the two were photographed making out on Tuesday, July 17.

People magazine is reporting Stewart is "devastated" by her actions, with a source describing the incident as a "mistake and a complete lapse in judgement".

It seems that something happened between Stewart and Sanders judging by the reports from People and Us Weekly, although a source close to Kristen revealed "It’s not a relationship". "She wasn't having an affair with Rupert. It was just a fleeting moment that shouldn't have happened," adds the source. "She never meant to hurt anyone. She's a good person who just made a bad choice."

It has to be mentioned that Stewart and Pattinson were seen out happily together just last Thursday when they held hands exiting a Los Angeles hotspot.

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