Why do I get so paranoid when I'm out alone?

For the past year or so I've felt really paranoid. When I go anywhere public I always think that people are staring at me and laughing about me. I get extremely nervous when there are people walking near me. When I walk home from school and I see other people, I either cross the road so I won't go past them or make it so they won't see me. When I go to places like town, where its busy, I get EXTREMELY anxious,nervous,paranoid, my heart thuds so fast it hurts, my face goes all hot, and I struggle to focus, and I just HAVE to get away from it and when I do its like a massive sigh of relief. can someone please explain what this is?