What could cause sharp stabbing pain under left breast?

Rib dysfunction?: May be muscle tear, costochondral inj., pericarditis, pleurisy, rib dysf. The latter often presents w/ pain (often sharp or severely achy & worse w/ deep inhaling or exhaling.) See Dr. to rule out serious problem like heart/lung. If determined to be musculoskeletal, may be treatable by osteopathic manipulation or prolo/neuroprolotherapy.Then see good osteopath for assessment.