Weird armpit warmth

Hi all, I had my lumpectomy in June last year and finished radiotherapy in September. For the last few months I have had a weird feeling in the armpit on the same side that my cancer was on. It is a bit hard to describe but feels as though there is warm liquid running into my armpit then my armpit feels really warm on the inside, the skin itself doesn't seem any hotter to touch. This lasts for a few minutes then subsides. It is not constant it comes and goes and I feel a bit daft saying it because you can't possibly feel (blood?, liquid?) running inside your body. I would be really interested to find out if anyone else has experienced this and what it was. Thanks for any advice, Kim