Go see a chiropractor.
I had this problem with my lower back, and it turned out to be that my hip went crooked and was pinching a nerve.
He popped that shit straight /nh, and I was as good as new.
Could be that your hip is pinching a nerve in your hip rather than your lower back like it was with me.
Either way, chiropractors are relatively cheap and I'd think that it's worth checking out.