Many possibilities. This could be from something irritating the diaphragm from above or below, from the rib or the costochondral attachments, an enlarged and tender intraabdominal organ like the spleen, or could be referred pain from elsewhere. Answering your question accurately would require a full exam. Good luck.
Chest pain. There are several structures in the left rib cage area. These include your ribs, the muscles surrounding your ribs, part of the breast, your lung, diaphragm, stomach, pancreas and most importantly your heart. A history and physical will help determine what is going on. You should go see your clinician as soon as possible.