Is it normal to instinctively shy away from personal contact?

I have been this way since I was...I think in middleschool. I don't like when people come into my personal space.

I instinctively flinch when someone gets a little too close, but not so much that if they just brush past me I freak. More like a controlled rebellion against touching.

I honestly don't enjoy hugs that often. I back away from people who try, but then I shrug it off or just joke about it.

It has become a habit to just repulse when someone gets into my bubble. This makes relationships with family, friends, ect. kind of difficult. In front of my parents and relatives and such I don't do anything. I just sort of ignore it. I don't want to seem impolite.

I usually force myself to let people get close to me, so people wouldn't jump to conclusions.

Here's the wierd part. Sometimes, I WANT to hug someone. I WANT to be near someone and talk to them. But if I hug them, and they hug back, I back away again. (unless, like I said, it's to an adult or something) It's like the feeling vanishes when that person returns the affection.

I honestly don't get it, and was hoping someone could shed some light on my little predicament.