I've had a small ball underneath my left armpit and next to my left breast and I recently grew two dark moles on my left nipple and it's been very irritated, what is a diagnosis for this?

I've had a small ball next to my left breast underneath my armpit. My mom said it was probably a muscle or something so I ignored it. Then this week I noticed a pain in my left nipple and it started getting worse and it's very sensitive. Then I also noticed that I've grown two dark moles around that nipple with more growing. My right side is completely fine. What's a diagnosis for this? What should I do? Could this be a sign of cancer?