I Had Breast Cancer 13 Years Ago,w/double Mastectomy, No Problems.

I had Breast Cancer 13 years ago,w/double mastectomy, no problems. Now within the last yr. or so, my left arm has swelled up, bright red and hot. Do not know why this happens, I have taken antibiotics each time
and no one seems to know why this happens. It happened again today. Any ideas what to do next?
My Dr. doesn't know why this happens. I am 78 yrs old, in fairly good health, overweight.