LeAnn Rimes Sings Lady Gaga to Eddie Cibrian

Sing it, girl!

LeAnn Rimes and Eddie Cibrian popped into a karaoke bar, Tail Feathers, in Henderson, Colo. over the weekend to use the bathroom — but she couldn't resist hitting the stage. (Check out TMZ's video, above.)

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Clad in a pink jumpsuit, Rimes, 27, belted out Lady Gaga's "Poker Face” as her beau, 36, danced enthusiastically right in front of the stage.

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Not all of Rimes’ recent performances have been so upbeat.

On Monday, she crooned about breaking ex Dean Sheremet's heart while on stage at the ASCAP Music Cafe at the Sundance Film Festival. (The couple split last July following Us Weekly's exclusive report that Rimes was cheating with the then-married Cibrian.)

"This song is going to make me cry," Rimes warned the audience. "It's about love."

Lyrics include: "What have I done? I broke the heart of the sweetest man. What have I done? I broke the heart of the only man who's ever loved me. Who have I become? I hope you find someone new, someone who deserves you."

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Though one source tells Us Rimes penned the tune "during the height of the cheating" and it is "100 percent" about her ex, another insists it "was written several years ago before the situation with Dean and the cheating. I can understand how it can be interpreted to be about what happened."

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