I agree with couchtater about
seeing a gastroenterologist. I had a bad bout of gastritis after surgery in 2011, which caused a lot of stomach pain. I still get flares occasionally. An upper endoscopy would show if anything like that were going on. I had mine done just after my colonoscopy (the same day) and slept right through both.
A bout of C.diff., I believe, can also cause stomach pain. Have you been tested for that?
I visit a gastritis site occasionally, and many other stomach-related issues are often included in the discussions. It's not a healingwell forum, but I hope it's okay to post here (if not, moderators please remove):
http://www.dailystrength.org/c/Gastritis/forum
Another thought is a referral from your doctor to an internist (doctor of internal medicine). My mom was losing weight for unknown reasons and was referred to an internist.
Have you had an ultrasound and/or CT scan of your stomach?
Hope you get answers soon!