So I stumbled across Hjaalmarch Stormcloak Camp and was curious if they were friendly so I went up to talk to the Quartermaster. He seemed cool. He sold me stuff so I started to leave. As I was leaving they all started attacking me!! Two faced bastards!!! =P

were you wearing imperial armor? if you wear imperial armor and tell them to svck it they will attack you.

I'm guessing you had joined the Imperial faction before this incident?

You don't have to be wearing faction armor for the other faction to be hostile towards you.