Irritated Underarm Rash

I am a healthy, 25 year old female who found a small lump underneath my arm, but not on the breast. It was soar and seemed to be a sac of fluid or perhaps a cyst. I went to my doctor who moved it around a little and said I was fine. That was about a year and a half ago. I recently switched doctors and was told I should get an ultrasound just to be safe. It turned out to be excess breast tissue. Well, it's been about 4 weeks since then and now I have this irritated rash in the same spot. I haven't changed my deoderant or body at all lately, and I even switched out my razor to a new one. I stopped using deoderant and am trying not to shave it as often but it only seems to get worse. There are blemishes/pimples and the amount is growing each day. Instead of deoderant I have been using Gold Bond body powder to stop the itch and irritation. Could you please help me with what this is and how I can stop it????