Appendicitis. An upset stomach usually occurs in the epigastrium or upper mid abdomen. Appendicitis usually starts with right lower quadrant abdominal pain. It usually progresses from this area to the naval area and is accompanied by loss of appetite and sometimes fever.
Pain type & location. When I think of an upset stomach, I think of waves of cramping pain in the mid abdomen. Appendicitis may often begin like this, but then "changes", moving to the right lower abdomen, becoming sharp, severe, constant, and progressively worsening. Many people can localize it well enough by pointing to one spot with one finger.