Hi,
First of all please talk to your mom or someone close to you about this problem. I don’t think that the bump in your armpit area has anything to do with breast cancer, especially at your young age. In my opinion, this bump sounds like it could be something as simple as a cyst or inflamed hair follicle caused by shaving or use of antiperspirants. It could also indicate a swollen lymph node that is often caused by bacterial or viral infections somewhere in the body, so please talk to your mom so you can go together to visit your family Doctor for evaluation. If you have an infection you might need antibiotics so this problem can be resolved okay?
Take care sweetie …