lol the OP has probably offed himself by now, you guys are mean. In a generous, titty-posting kinda way.
But to the OP- I'm a chick, and most of the time that I've been on BC, I wasn't having sex. I started taking the pill for a full year before I even had my cherry popped and I kept up on my pill when I was single and not getting any cock. Girls take it for more reasons than just pregnancy prevention - to regulate periods, to keep cramps and bloat down, to help acne, to help with hormone imbalances and mood swings etc etc. Don't be a douche and assume she's cheating on you simply because she's taking birth control. Would she have the right to go off on you if she saw that you had some condoms in your bathroom cabinet? I don't think so.