i was that way all through high school. it's not good to give in to it...though i know it's much easier. it turns out i missed out on a lot in school by letting this control me. but my point is i'm 24 now and things change a LOT from your age to my age. now i'm not nearly as uncomfortable in social situations as i used to be. i'm also not nearly as uncomfortable as i used to be with talking to other people i don't know. anyway...what i'm getting at is that i kind of wish i'd pushed myself into more social settings and tried harder to make friends while i was in high school...even though it would not have been comfortable for me. like the person above said...face it head on. don't let it control you. if you have a fear of what people 'think' of you...i used to have the same fear. but everyone gets to a point...probably in their early 20's or so...where they realize it simply doesn't matter what people think. you will always have people around you who want to be around you and they are the only ones that matter.
It may be a social anxiety problem. If it really bothers you I would get it checked out. This problem, along with other phobias, are actually helped by facing the problem head-on. It seems as if you've developed agoraphobia, which simply means you are avoiding the situation because you are afraid it will create anxiety. Try to PUT yourself in these social situations, even it if does create anxiety at first, so you can prove to your mind that you are physically capable of participating in them. Hope this helps.