Since it is under your breast and on top of your rib, I would question if you have a rib fracture as my recent one felt a lot like that. But in any case, breast tissue can extend down that far, and as far up as your shoulder - so it really could be anything. Your best bet is to see your doctor and have some xrays, ultrasounds or even a PET CT scan done. Those tests there can produce some very accurate results. I hope that helps some, if not please feel free to ask more questions!
Angi