It a culture thing, sounds like.
It's fine to pick your nose in the bathroom when no one is watching (where you can wash your hands after...) and another to do it in front of everyone else. You probably have felt the negativity from others about your actions, same reason most people get self conscious or embarrassed if they pass gas in public.
Therefore, you try to conform. You can't "be just you" while out, there is alway a public face and a private face. You wouldn't go to class and lounge in your boxer-briefs (or heaven forbid, tighty-whities ), pick your nose, and pass gas in class, would you?
To an extent, be yourself, but if there is anything you just don't feel comfortable doing in public, don't do it. Of course you're not going to be comfortable doing things that other's ridicule you for, but there is a point where you just have to say, "I won't let other's run my life".
Maybe if you watched yourself in mirror to find out exactly what they are talking about, you could find ways to adapt things that would help you relieve stress and not make you look "weird" to others? (relieving stress in a way that everyone around you is going to pile more stress on you isn't going to work too well.)