I have a dull but very noticeable pain in upper left abdomen just under my ribcage. what could it be? it's much worse while sitting.

Many organs there: Stomach, spleen, pancreas, splenic flexure of the colon. Inflammation of any of these can b the culprit. Gastric causes would b the most common. Gastritis, ulcers, even gerd can refer there as well as duodenal ulcers. Pancreatitis is not that common. Make an appointment 2 c a gastroenterologst 4 an eval. Also u can try liquid antacids while u wait 2 b cn.It may help.