Rachie,
I know how you feel, and it's the pits!! I feel that way sometimes too. I'll go out with my friends and then after a little while it's like I hit some kind of brick wall, and all the energy leaves me and all I want to do is get home and be by myself. I also feel guilty because this will happen with my friends from high school, who I only see a few times a year, and I feel horrible afterwards for "wasting" the time I had with them. It makes it really tough to be sociable because I suddenly feel really disconnected from the rest of the world.
Do you have a counsellor or a therapist? I found that telling mine about what was happening really helped, and she's been helping me not to worry about it so much and slowly make my way back into proper sociability. Maybe yours could help you too!