I think that you should look into getting some help. I'm not saying you're crazy but i think it would be a good thing ti investigate this paranoia with a therapist of some sort. Also to discuss your low self esteem which can often be the cause of the paranioa.
If you don't have a gp the first step would be looking into getting one. And that shouldn't cost anything, nor does most type of therapies cost you anything as long as you're not going private.
I understand you're feeling scared but i really think it would help you to find someone to talk to about these feelings you're experiencing. As i said earlier, i don't think you're crazy but i think a low self esteem and the worry about your looks can be the cause of some of your problems. When i think too much about my looks (i'm overweight) i also keep thinking that people are looking at me and jugding me and thinking bad things about me.
I also want to say that i have seen pictures of you on the piccie board and i don't think you're overweight. Actually you seem very slim

And i think you look really good with your funky hair and all the piercings. I wish i looked as good as you!!!!
Try and register with a gp. Maybe your parents can help you with that if you feel comfortable. There is a lot of help out there and you should be able to find someone who can help you. It is a bit scary, yes, but it can really make such a huge difference.