So sorry that you are still having so much pain,but this is not an uncommon problem. It's a common phenomenon in at least 25 percent of women.Generally, when they have this syndrome, women will feel a lot of pain, numbness, and burning under the armpit and also across the chest. Most of the time this occurs because of injury to a nerve. Commonly, there's a nerve called the Intercostal Brachial nerve, which is injured during surgery.
These sensations can last for a long period of time, and can actually occur anywhere from weeks after the surgery itself, to years later. Women who experience this pain should be seen in a pain center,because there are some nerve blocks that can be done. In particular, one that is sometimes helpful would be the Stellate Ganglion Block, which would be done by an anesthesiologist trained in pain medicine.
The other thing worth thinking about and considering ,would be acupuncture,many women seem to find relief rather quickly.
Best wishes and I hope that you'll feel better soon.