I do not have enough information regarding what kinds of tests were done, nor your age, or gender for your doctors to determine your condition. You know your body better than anyone else and obviously you feel something is wrong.
First of all you should have a good physical exam and history to see if you have any fever, low blood pressure, changes in bowel habits, any new food intake and nutrition history.
Next, a CBC, CMP, lipase and amylase blood testing to rule out any infectious condition (appenditis,
cholecystitis) or
pancreatitis.
Next, you should have a CT scan of the Abdomen and Pelvis to see if anything is amiss.
Next, if these tests are normal you should have an
upper endoscopy to look at the
esophagus and stomach to see if there is any
gastritis or ulcers.
I hope this helps, best regards