Stewart, 22, is said to have had a brief fling with Rupert Sanders recently. Sanders, 41, directed her in 'Snow white and the Huntsman', the People magazine reported quoting sources.
British director is married and has two young children with model Liberty Ross.
"Kristen is absolutely devastated. It was a mistake and a complete lapse in judgment. She wasn't having an affair with Rupert. It was just a fleeting moment that shouldn't have happened," a source said.
"She never meant to hurt anyone. She's a good person who just made a bad choice," the source added.
Stewart and Pattinson, 26, were photographed together just days ago at Comic-Con. The British actor had even hinted about a future marriage.
The couple first met on the 'Twilight' set and reports began almost immediately that they were a dating. After being initially coy, they became open about their relationship.
The representatives of both Sanders and Stewart were not available for comment.
Meanwhile, Us magazine has claimed that they have photographs of Stewart and Sanders together.
