This is so high school . . . oh wait you are still in high school.

j/k man. You need to ask her. Obviously she is going to this thing. Maybe some dude asked her before you two started going out :dunno: Either way you need to find out. I think it is ****ed up if she made plans w/ some guy, even if it vwas before you two started dating that she wouldn't tell you about it. If that is the case you need to re-evaluate your relationship. She could also be the type of chick that assumes you'd go. Either way, communication does not to be her stong suit.