Bisexuality Support Group
Bisexuality is a sexual orientation which refers to the aesthetic, romantic, and/or sexual attraction of individuals to other individuals of both their own and the opposite gender or sex. Most bisexuals are not equally attracted to men and women, and may even shift between states of finding either sex exclusively attractive over the course of time.
This will give her a chance to take ownership of her "mistake". Hopefully it was a mistake. Don't say anything about it not being a mistake, but keep in mind that she may be someone you can't trust. Give her a chance to earn your trust back, but don't let her get behind you with a knife anytime soon.
That's your choices, they both have their pro's and con's. It appears that no real harm has been done. At least no one else has said anything, no one has disowned you at this point. So even though you have every right to be upset and hurt, don't take it too seriously.