Pain on left side abdomen under rib when sitting when i lay flat it goes away?

Position: Any pain related to position sounds like musculo-skeletal in origin, not visceral. So you might want to try looser clothing, change chair position, that sort of thing. Area under the left rib cage means area of stomach, pancreas, spleen, diaphragm, so if it persists, these things need to be looked at. Also, it could be atypical angina, pain from heart muscle not getting enough oxygen.