Armpit lump

I've had an left armpit lump for a month now and recently saw my doctor about it. He confirmed it wasn't a swollen lymph node and was also concerned about some lumpiness he felt on my left breast as well. The lump is deep under the skin and he had to press into my skin to find its borders. I am scheduled for an ultrasound and mammogram next week. A day after the exam, the lump seemed bigger and I started having random shooting pains where it is. Yesterday I woke up and the area was much less swollen (noticeable so) but I could still feel the lump when I pressed into my skin and it was pretty sore. Today it's back to swollen and slightly painful.

Does a cancerous lump behave like this or is it more likely to be a cyst? My ultrasound isn't for another week and I'm really nervous with the random changes in my armpit.