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.
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.