Other reasons. Can be pericarditis, or inflammation of the sac around the heart... Could be pleurisy, or inflammation of the lining of the chest cavities. Could be musculoskeletal, including pain related to the cartilage connecting the ribs to the sternum (costochondritis) e.g. From excessive coughing. Could be a tumor. Variety of reasons that could be narrowed down based on characteristics of the pain.