Do I have social anxiety?

I'm always quiet and shy. At school I sit by myself and don't have any friends. When people do come up to me I get really nervious and can't think of anything to say and when I do say something it's really stupid. I end up thinking about conversations I had for days and hate myself for doing something dumb or not saying something. And when I go places or at school I always think people are looking at me and talking about me. I'm even nervous with ppl I known for years. Im going to college next year and I really want to quite this :(