Insiders allegedly told People the 22-year-old actress is "absolutely devastated" with regret about her roll-in-the-hay with Sanders, a 41-year-old, married father-of-two.
"It was a mistake and a complete lapse in judgment," a source told the magazine. "She wasn't having an affair with Rupert. It was just a fleeting moment that shouldn't have happened. She never meant to hurt anyone.
"She's a good person who just made a bad choice."
People also reports a rep for Sanders could not be reached for comment and that a rep for Stewart had no comment.
The Twilight Saga: Breaking Dawn - Part 2 hits theaters November 16.
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