so what is your plan of attack, and does she know that you know?
b/c you really do have many many options here, some better than others. let's review some:
a) cheat on her, since it's only fair
b) confront her and rip her to shreds making her feel like a piece of shit
c) don't tell her you know so that you always have this trump card in your pocket and pull it at the right time down the road
I don't know your situation with her, and that sucks for you bro, but, please do not be a p-ssy. do not let her get away with it.
but, if you are truly in love with her, truly believe that it was a freak occurence, and am the type of person that can forgive and forget, maybe you should stay with her.....
or just punch her in the face.