okay, i'm a hippie, but: if nothing physical has shown up with repeated testing, maybe it's psychosomatic. this guy john e. sarno is pretty amazing.
http://www.amazon.com/Healing-Back-Pain-Mind-Body-Connection/dp/0446392308the basic argument is that the mind tricks your body into feeling physical pain in order to distract your conscious mind from dealing with emotional pain. i dunno. i've seen it happen.