My stomach at the left side keeps going hard and sore, why?

LUQ anatomy: Anatomically, the left upper quadrant (luq) of abdomen includes: spleen, pancreatic tail, stomach, descending duodenum, splenic flexure of the colon, associated vessels. Don't forget the diaphragm, heart, and lower chest are pretty close by. The abdominal musculature, ribs, and skin can all contribute as well to localizing luq symptoms as described.