How to react when a good friend does something bitchy?

I have a lot of friends, but only a few that I really trust. I was just on Facebook, and I noticed that one of my few good friends that I trusted, just had a birthday party. I wasn't invited, and I'm really hurt. I'm not the type of person to be on the outskirts of a group, but I've been applying myself a little more in school lately, and in turn it has taken a toll on my social life. This is the 2nd party this month, that was hosted by my so called "friends" that I wasn't invited to. The first one didn't bother me to much because I wasn't as close with the girl, but I thought that this girl was one of my best friends, and she invited people that I know she doesn't even like. She is one of the nicest people in the world, and I don't really want to confront her because I don't want to start anything up. I know she didn't exclude me purposely, I'm just kind of wondering some things I could do, to get back into my social group. Ever since I have began applying myself in school, I have felt out of the loop socially. And after the first party I missed, I put less effort into school so I could get back with my friends, but it has been kind of hard to break my way back into the "in" circle of my group. Any advice on how to get back in, or how to deal with my situation on being left out of one of my nicest, best friends partys? Pleasee Helpp!!! Thankss sooo muchh!!!!