I take it you're not a massive recreational drug taker? (Just to clear that one up first because if you are I think I see the problem and the solution is pretty easy).
Paranoia is usually a sympton of something else in my experience. I can get quite paranoid if I'm feeling particularly low. The fact that when you're low and you're vulnerable and probably giving yourself a very hard time, you don't have much of a perspective to realise that other people can't actually see right through you to what's going on inside your head.
Of course there is the flip side of the coin which is if someone does notice you for long enough they will make a judgement on you, we all do it all the time with people we see. It's generally a conscious thought instantly loss and quite shallow, nice shoes, that outfit looks dreadful etc...
Ultimately, mostly, people just don't care or notice enough aobut others to really be watching or judging you for any length of time, you're just projecting. Best thing to do when you catch yourself having these thoughts is to turn them around, either look at why you're thinking them or just take a deep breath and close your eyes til the wave passes. If it gets to much, see your GP.