I too had a new lump form close to the chest wall 9 months after a mastectomy on my right breast. At first they thought it was scar tissue at the end of the incision. Biopsy confirmed cancer. After a lumpectomy to remove the new spot I had 12 sessions of chemotherapy and 25 sessions of proton beam radiation at Mayo in Rochester. Oncotype score did not show I needed chemo or radiation after mastectomy. I had no lymph nodes removed. I was prescribed anastrozole after mastectomy but then Tamoxifen after lumpectomy, chemo and radiation. Was on Tamoxifen for two years but due to side effects was switched to Letrozole. I have been taking it for a year now. Still some side effects but managing them. Din’t know why this happens but am grateful it was found early. At the time a PET scan was also done and it showed thyroid cancer. Had half of my thyroid removed the day after my lumpectomy. Grateful to be able to say I just had my three year follow-up and all is well. I have lost 50 pounds in the last three years with healthier eating and exercise. I am so blessed to have a great team of doctors at Mayo! Take care. Stay in touch.