Joe,
My father is actually an expert in this type of field, back pain that is. I told him about your situation and he explained to me that it could have a lot to do with a build up of tension in the lower body area. He said the best thing you can do this to lay on your back for 30min to an hour a day and have your significant other perform oral sex. The more times a day the better it will be. Kind of weird I know, but he showed me all kinds of proof.
Good luck.
(Show this to your girlfriend. But not this part. Your welcome.)