My friends meet up without me "/?

Well soon it will be summer and my 'friends' don't invite me out anymore at all. Last summer we had such a great times and everything and it was great now, they never invite me ever ! i have done nothing to them. It's too late in the year to make new friends for the summer so what do i do ? turn around and say 'Hey! what is your problem?' ? but i can't see that helping me! The thing that bugs me most is that when i am at school they like come running up to me to say hi and all and like the leader of the group if i see her in a corridor she will always smile and say hi. I am just really stuck on what i should do and need some advice!