Don't Know What Happened...

I just lost about 13lbs the last term of my school year. I come home, my brother is getting married, and I gain 5lbs in two weeks! I'm so frustrated, and truth is, I don't even know how it happened! First, its just going out to dinner, then its oh its ok to have one desert....it all adds up, and in my case the adding equals five pounds in about two weeks! The wedding is tomorrow and now my brides maids dress is too tight and I feel I'm just going to keep gaining weight until I'm right back to where I started! I have less than 5 weeks of summer, until I report back to school for preseason. I'm going to do my best to drop those 5lbs, any support would be very much appreciated!