Social anxiety can creep up on anybody, in my opinion, so don't get down on yourself about it. There are many things you can do like Mammo has said. Have you ever considered counseling or maybe even groups that help with public speaking? Many people are uncomfortable in some social settings, but you can definitely confront your fear...keep us posted!
You need to speak to your parents about his so that you can get help. Also, try talking to your school counselor. What started out a nervousness sounds like it's turned into a lot of anxiety. Just tell your parents what you've told us, they love you and don't want to see you feeling this way.