Having Pain In Left Breast. Cause?
Given how you describe this pain, I do not think that it is coming from the breast itself. It sounds like the pain is deeper than the breast tissue and more likely related to the chest wall.
At exactly the location that you are describing, some people develop pain and inflammation at a junction on the ribs called the costochondral junction. This is a common source of pain related to 'costochondritis'. This would fit with what you are describing. Most of the time it is treated with 'ibuprofen-like' medications to decrease inflammation. Often it is helped with Naproxen 500mg twice daily.
This is something that many women would try, but if you do this, it might also be good to get in to your doctor for a full exam and breast exam just to be sure.
I hope that this helps and good luck!