a real shot in the dark but I wonder if it could be a muscle in spasm probably the psoas muscle ......... maybe try and find a good trigger point therapist or chiropractor

I tried some googling about psoas mucle but it has become a bit trendy to refer to it in yoga and pilates and I couldnt find a good precise site for you in the time I have now

but several years ago after reading a post on OBT about stomach pain being related to the psoas I found lots of info about how it can cause all sorts of problems and was able to trace my own problem to an exercise I was doing agrevating my psoas muscle

good luck
koz