Unexplainable Blister

I keep getting what looks like water blisters on my left leg near my knee. It looks just like the kind of blister that you get from a burn. Only one appears at a time.  It fills up with liquid, pops, and eventually scabs up and heals. It takes months to heal. Then a month or so later I get a new one in roughly the same area. Same routine - blister, pops, leaks, scabs and then eventually heals. I asked primary care doctor and he does not have a clue. I have Stage 4 Small Cell cancer. I asked my Oncologist if the blister could be caused by the Cancer or perhaps the Chemotherapy or medications I am on for treatment. Oncologist had no idea. These blisters only appear on my left leg and are usually just around the knee area. Additionally my left leg has nerve damage (completely numb from groin area to toes) and is atrophied (significantly thinner - especially around knee, than right leg). I am sick of going to the doctors and getting a blank stare and hearing "I don't know why". It is even more irritating when the bill for comes in a few weeks later. I hope someone out there knows of someone who has or had a similar problem. Better yet if someone has any idea what could cause it and I can run it by Primary Care doctor and get the correct diagnosis and treatment. Please Help
Gary G