The drainage is a normal part of the healing process. If she does not have the JP drain stuck in her chest than she should consider herself lucky. I had a bilateral about 3 months ago and had to wear these drain tubes in my chest for 2 weeks. The pain she is experiencing is a result of both damaged nerves and muscles. My pain was so severe that I needed prescription pain killers for about a month after surgery. Physical therapy and massage therapy have given me back my range of motion and made life almost painful. I suggest everyone who has had a mastectomy invest in these therapies. I wish the best of luck to her.
I had the same problem with fluid buildup under my arm after a lumpectomy and axillary node dissection. I had a drain in and the buildup occurred after the drain was removed. I had to go in about 5-6 times to have the fluid drained before it finally stopped being a problem. It can be extremely uncomfortable since it forces you to hold your arm in a position that isn't relaxed.
Dear TearyThunder: It is likely that the pain she is experiencing could be from the severed nerves but pressure from fluid can also be a reason for discomfort. Fluid can form as a result of trauma to the area and disruption of the normal tissue drainage system. Did your sister have a drain after the mastectomy? Sometimes a person may need to have a drain placed until the fluid collection subsides. She should discuss this with her physician.