I Was Diagnosed With Breast Cancer 12 Years Ago And

i was diagnosed with breast cancer 12 years ago and had a left masectomy. 5 yeas ago in my neck lumps were discovered well under the skin and when biopsied came back as the breast cancer, I had radiation and all was fine and lumps disappeared. Now on my back - again left side i have a cyst like that has developed which I have had for 18 months at least. It is definetly in the skin and sometimes red as right under bra strap. It is not painful but hard and at top of skin. It is smooth around lump