Hi, 2009 I Had Breast Cancer, I Had A Mastectomy

hi, 2009 I had breast cancer, I had a mastectomy but they didn't do the good breast, I am still on tamoxifen and have been told it will continue for 10 after my diagnoses. My problem now is, for several months I have been getting an itchy breast/nipple and it has been aching alot and recently I have been getting pain under the arm next to same breast. I had a mammogram and ultrasound the other day and nothing showed. The next morning I woke to find a itchy rash on the breast above nipple and a itchy rash on my back straight behind where the breast would be. It seems to be spreading. I am not due to go back to the oncologist until 22nd January. Could this rash,aching and itchiness be related to more cancer. The pain I am getting under the arm is sharp.