Try Zim's Max-Freeze for sore muscles and joints. It's made with natural ingredients and you rub it right on where it hurts. It's at the drug store and it really works!! Good luck!
How long have you had it? It could be a lot of things: pulled muscle, disk, or even a kidney infection or stone. Do you have any other symptoms? Fever? Any pain in the leg? Any trouble urinating? Have you tried any OTC things for the pain? Ice, heat, ibuprofen, Tylenol? Do they give you any relief?
I think if you give a little more information you may get some better suggestions.