I am I so quiet and uncomfortable around a small group of friends?

When I'm around a group of friends, like 3 or more people, I tend to like..freak out.. in a way. I start to get really quiet and sometimes pissy. After a while, like at a sleepover, I feel like I have to just get away. A lot of times I will just run off and go hide somewhere just to be alone and my friends come looking for me, but I just pretend I was playing a prank, when really I was like freaking out. Sometimes my heart will start racing, and then I start crying out of nowhere and things race through my mind and I keep thinking that everything is wrong and end up blaming myself. I just get pretty uncomfortable around a group of friends. Everyone asks me whats wrong and I dont know what to say. What is wrong with meeee!