Been there, done that, and bought the tee-shirt.
Welcome to some really cool rush of feelings then the crash and burn. Personally, I would ignore her, and move on. Sometimes this makes her realize that you have another life also. She could have been toying with your feelings by visiting with the other guy, trying to get you jealous, but if she indeed did sleep with him, that is going a bit too far to create jealous feelings from you. Other thoughts, she may asked you to go to the party so that she didn't go alone and look desperate.
Just keep stepping up to the plate and you will find the right one.