Confidence issues? ? Help?

Okay, basically alot of people say things about me as a joke, but if it's about my appearance, I'll laugh it off but feel like crying, I have no confidence and when this happens I go really quiet and I try to hide my pointed out flaw and when I walk I almost limp for no reason? Why!? Like once my teacher started shouting at me for no reason when prior to that my sister had pinned me against a wall and hit me really hard and I had a panic attack in class, and then every time some one laughed I thought it was about me, help?