Things are about to get very ugly for Kristen Stewart and Robert Pattinson.

This week's UsWeekly cover story tells the tale of Stewart cheating on Pattinson with her "Snow White and the Huntsman" director, Rupert Sanders . The magazine says it obtained incriminating photos of the 22-year-old actress and Sanders, calling the moments caught on camera a "steamy rendezvous."

Sources tell People, "She wasn't having an affair with Rupert. It was just a fleeting moment that shouldn't have happened. " The "rendezvous" allegedly occurred recently and, to add insult to cheating injury, Sanders, 41, is also married -- to British Vogue model Liberty Ross. The couple has two children.

According to GossipCop.com, there was some romance between the two, but whatever happened was just a fling and not serious. "She made a mistake," a source told the site. "She feels awful.

Stewart met Pattinson on the set of "Twilight" and the couple quietly dated for months before it became impossible to keep their relationship a secret. The pair have been spotted out together since Stewart's alleged affair and attended the Teen Choice Awards together on Sunday.