By People Staff
Updated August 07, 2010 05:00 PM

Surprise, it’s Lady Gaga!

The singer, who just broke a VMA record by earning 13 nominations in a single year, made an unexpected appearance on stage with Semi Precious Weapons at Chicago’s Lollapalooza music festival on Friday.

Gaga came on stage and played drums during the song “Magnetic Baby,” then joined frontman Justin Tranter for some crowd surfing.

“If anyone calls Lady Gaga a pop star I’ll f–ing kill them,” Tranter said. “She is a rock star and she is helping Semi Precious Weapons bring rock and roll back.”