I have exactly the same problem...it drives me crazy as I cant seem to get comfort or relax it.

I have come to these conclusions.

POSTURE - Maybe I have been in such bad posture for so long that my muscles have cramped or developed in the wrong way.

TOXINS - pretty much from food to mercury and anything

DEPRESSION - Since I find I was taking SSRI's and the feeling in my gut was mroe uncomfortable and heard serotonin is made form the gut too.