Why am I so antisocial?

During the school year, I'm as happy and as outgoing as I can be. But, then, as soon as summer starts, I feel like I just shut myself off from the rest of the world. It's such a weird feeling, because I don't want to be around anyone, yet I'm so lonely...I hate it. I'm so confused with my feelings. I don't want to be social, but then seeing other people be social and happy makes me even more antisocial and upset. I don't understand...please help me understand how I'm feeling, and give me some guidance to get out of this slump. Thanks in advance :)