Pain, as you have described, can be from muscle, nerve, rib, the lungs and the lining of the lungs. Sometimes a good physical examination can help in deciding which of these is causing the problem. Sometimes, other tests are needed to differentiate the cause of the pain. I would speak to your doctor about the process of determining the cause of the pain.
Consider costochondritis as the diagnosis, especially since the left side just below your breast is one of the chief presenting symptoms.
I hope you feel better soon.