What would cause pain on upper left side of body near abdomen under ribs?

Left apain: The pain can be muscular, colitis, costochondritis, pneumonia can cause pain in lower chest with deep breaths.

Several things: Upper left abdomen contains spleen, tail of pancreas, splenic flexure of colon, part of stomach, left kidney, left lower ribs and muscles. Costochondritis, strained muscles, trauma to spleen, stomach ulcer or tumor, pancreatitis, problem with left lobe of liver, left kidney infection or stone, left lower lung infection, splenic flexure distention with air from irritable bowel syndrome, to name few.

Splenic flexure : Many things as already mentioned, but there is something called splenic flexure syndrome where gas is caught in that area causing discomfort. It would be best to be checked out for the possible cause.