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.

After working so hard to keep their relationship private, Kristen Stewart has issued a very public apology for cheating on her boyfriend Robert Pattinson.

she said : "I'm deeply sorry for the hurt and embarrassment I've caused to those close to me and everyone this has affected. This momentary indiscretion has jeopardized the most important thing in my life, the person I love and respect the most, Rob. I love him, I love him, I'm so sorry."

Pattinson's reaction to Stewart's infidelity remains unknown, but an April interview he did with Italian Vogue might give some hints.

Stewart and Pattinson met four years ago while filming the first Twilight film. Though rumors began immediately that the co-stars were more than friends, the couple has only been officially dating for three years.

Over time, the pair has become much more public with their affection, even attending the Teen Choice Awards together only a few days ago.

Do you think this is really the end for Robsten? Or will he give Kristen another chance?

I really hope Kristen and Ropert stay together forever ..