Ah! It's so automatic for me now that I forgot that exception... I hate people behind me, too! If someone is near me, I have to be able to see them. I don't even like someone to hold a door open for me because you have to walk in front of them to get through the door. Just someone standing behind me is pretty bad, but if I'm walking and they're walking behind me... just being in motion... I get this weird tension and I have to make them walk in front of me, or else I'll get in a weird mood and look behind me to see them every few seconds.
Verdandi, overstimulation makes more sense. Lately I've been trying to think of things in terms of feelings as reactions, so I overlooked sensory when I was thinking about it. Even though it doesn't bother me the way you've described that it bothers you, I do get strong sensory feeling when people are near me. Can't explain it too well, but it's a sense of their presence, a kind of pressure? Not the same kind of pressure as actual physical touch.