I think my girlfriend cheated on me...with another girl?!?

My girlfriend is kind of a tomboy, like, she's into video games and stuff, but i don't think she's a lesbian. but i heard a rumor fom one of my guy friends that she cheated on me with another girl. im so torn apart right now. i really loved her and i can't believe she would cheat on me. (i'm not a homophobe so the fact that it was a girl doesn't matter THAT much, but still, im pretty pissed off.) so what should i do? i don't have any proof, do i confront her? break up with her?