Girlfriend is in shape!

Britney Spears headed to the gym in Calabasas today, and the pop princess was looking mighty fit in a tank top that exposed her muscular arms and toned abs. Maybe she wanted to work off some aggression after Christina Aguilera mocked her singing abilities on Jimmy Fallon last night?

Maybe she's just trying to keep her bod slim and trim for boyfriend Charlie Ebersol? After all, a source dished to In Touch that the couple's relationship was "fueled by sex" and "he absolutely rocks in bed." TMI alert! Although, Brit Brit is probably enjoying her romps between the sheets more now that she's not with her pudgy legal assistant ex beau David Lucado!