spin the whole situation on her. Take her out and tell her you have a girlfriend so you can't be all over with her. If she's really into you, it'll only drive her all the crazier for you.

And if she isn't, and thinks you're a player now, then to hell with her and move on. (bonus: if you don't feel like being 'the bigger man' tell her she wanted to see you behind her BF's back, so she's just as guilty)

Either way it's not your problem.