Left-sided rib pain may be caused by a tumor. Some patients have tumors in the lungs or pleura, which grow and compress the nerves in the patient’s left rib cage, causing the patient to develop this condition. Other patients have muscle inflammation. Problems with the latissimus dorsi, pectoralis major, triceps and other muscles may lead to left-sided rib pain. Patients who overuse their upper extremities (such as throwing a shot put) and strain their ligaments may also suffer from left-sided rib pain. Rib pain after a cold may be due to costochondritis, which is mostly self-healing.