I think she didn?t consider you are as a boy friend, because you guys only dated for a week. And, it could be that you didn?t let her know that you want a more serious relation ship other than a one nightstand. It is very common for both girls & boys to ?play the field? to get it out of their system before they get into a serious relationship with someone. (Or, because they lost faith in relationship since they just walk out of a bad one, and just want to have the opposite sex surrounded them to make them feel better)
You could discuss your feeling with her and see what she have to say regarding what you saw at the party & why she did it, because you could learn something from the experience even if it is painful/embarrassing to do so. And, if you strongly feel what she did is not okay in your book, then you could just be friend and have gain some knowledge out of the experience. But, it is not okay for you to spread hurtful rumors or say nasty things about her to her or others, and it will make you look stupid in the eyes of others especially the opposite sex.
It is easy to lose a friend than make enemy.