When Jason Wahler and his fiancée, 19-year-old USC sophomore Katja Decker-Sadowski, announced their engagement at an August 25 housewarming for their L.A. pad, one reveler wasn’t too psyched: his ex Lauren Conrad, whom he dated on and off for a year.

Despite her congratulations, “it seemed like she wasn’t happy for me,” Wahler tells Us, adding that LC showed up unannounced with Audrina Patridge and the Hills cameras.

“I wanted her to go…There was a weird tension,” says Wahler, 20. “I have to give up a friend until it mellows out.”

For her part, Conrad, 21, tells Us, “I am really happy for Jason and wish them all the happiness.”

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