Goes without saying I first and foremost recommend going to the doctor (or finding another, if you can, because it's not something you want to put off if it's something serious.
But - I will say I have had some terrific stomach pains that ended up being related to the fake sugars and similar things that are in low-fat and diet foods. So, anything that might contain any of those sweetners (splenda, equal, sweet & low, etc), or similar products might be something worth avoiding. I cleaned up my diet of those and any foods that have them, and it's like I have a new stomach.