Sounds like you need to get an ultra sound on your entire abdomen area, including stomach, gall bladder, liver, kidneys, pancreas, etc. to figure out what is going on. If that turns up ok, then you probably need a colonosopy, and if that turns up ok, get an MRI on your back.
I agree with demtlard. You need a CT or ultrasound of the abdomen. Both are pretty painless. Then you go from there is those tests are negative.