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Oh no! Could there already be trouble on the horizon for Kristen Stewart and Rob Pattinson!?

As you know, Kristen is here in LA shooting The Runaways while Rob is in New York working on the romantic drama Remember Me, and Star mag reports that Kristen is "in an absolute panic" that she's going to lose her sexy lover to his new co-star Emilie de Ravin!

A source tells the mag, "Kristen's been calling him nonstop since he's been in New York and gets uneasy when he doesn’t answer. She’s nervous he'll fall for Emilie. She thinks it's only a matter of time before he hooks up with another girl."

But we thought Kristen and Rob were madly in love?! Well, they may be, but Kristen may also have a good reason to be concerned - Emilie split with hubby last month, and the blonde beauty has a history of going after her costars, even if they’re involved with someone else! Star reports that while on the TV show Roswell in 2000, Emilie hooked up with Jason Behr, who was then dating castmate Katherine Heigl! Talk about a man stealer!

"Rob could be Emilie's rebound guy," says the source. “He’s her type, boyish, with chiseled features." We certainly hope Rob stays with Kristen and doesn't fall for Emilie...as gorgeous as she is, we're pretty partial to K. Stew!