You could be experiencing pain from scar tissue, from poorly draining lymph nodes, from infection and/or inflammation, as well as the possibility of recurring tumor. Get your dr to give you a thorough exam, and keep pushing for satisfactory answers. If you don't feel you are getting a reasonable explaination and proper treatment, get a referral for a 2nd opinion. As the consumer and the patient, you DO have the right to consult a different physician - and any dr worth his or her salt should not hesitate to provide you with a referral so that your insurance will cover it. Also, you can ask your dr to refer you to a physical therapist well-experienced in treatment of post-mastectomy recovery. This could include massage, physical manipulation, stretching, hydrotherapy, exercise under supervision and a home treatment plan.
After a mastectomy, lymph nodes around the surgical site can get congested with crystallized drainage/secretions. Many women report stringy bumpy areas at about 10 months post-op - this is crystallization due to inadequate drainage. These crystals can be broken up through gentle massage of the affected area.
Pain from post-surgical scarring can also be eased by gentle but persistent massage of the affected area. Doesn't have to be anything fancy - doesn't even have to be prescribed by a dr or therapist. To the extent that you can stand, gently but firmly massage the area at least several times a day - gentle pressure may at first be somewhat uncomfortable, but within a short period of time SHOULD help the area to feel better. If gentle massage causes onset of moderate to severe pain, stop and get another exam.
Unless you have another physical condition which affects your balance or mobility, You might also consider using a rebounder (mini-trampoline.) They can be purchased fairly inexpensively, and gently bouncing for just 3 minutes twice a day can greatly improve lymph drainage, decrease lymphedema, help to break up scar tissue, improve muscle tone and circulation, plus it's a lot of fun.