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Nikki Reed's late-night rendezvous with Robert Pattinson, her hunky co-star in Twilight and New Moon, in Vancouver, British Columbia, on March 29 ended with her flying home to LA the next morning. But distance didn't seem to make Rob's heart grow fonder. Since Nikki's return to the New Moon set in Canada on April 8, Rob has been playing it cool with the actress as he continues to explore other romantic prospects -- including Nikki's onetime BFF, Twilight leading lady Kristen Stewart, whom he'd like to spend more time with. "Rob's been flirting with everyone on set in front of Nikki, even though they've been sleeping together," an insider says. "It's causing tension."

While the duo were on the same page when they began their passionate affair on the set of Twilight, things have changed for Rob as his star has risen. While he still views their fling as a casual hookup, Nikki hopes it will bloom into a full-fledged romance. "Rob knows Nikki's not the one for him, but he still goes back to her and leads her on," the insider says. "She thinks he'll come around and they can be a legitimate couple, but Robert is determined to enjoy his new fame -- and all that comes with it."