Diverticulitis typically causes pain in the left lower quadrant, not around the stomach area. Peritonitis presents with fever, as well as fluid within the abdominen.
At this time, you can consider an abdominal CT scan to evaluate for the possibilities mentioned above. If there is fluid in the belly (i.e. ascites) this can be tapped to evaluate for the possibility of peritonitis.
Stomach distension and upper abdominal pain can be caused by GERD, an ulcer, or inflammation of the upper digestive tract. The most comprehensive test to evaluate this would be an upper endoscopy.
More specialized testing can be considered with a gastric emptying scan to evaluate for gastroparesis.
These options can be discussed with your personal physician.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
http://www.straightfromthedoc.com