Why does this always happen when I'm around people?

People drain me. Like, really drain me. I can go all day without speaking to people just fine without my energy levels completely lapsing. But if I spend fifteen minutes just being around people I'm done for the day. At thirty I actually begin to feel physically ill. Nausea, dizziness, can't think clearly. My mouth goes numb. I feel like the swine flu slapped me in the face and insulted my mother. I feel awful. What's the cause for this? I'm usually not that nervous around people.