What Could Tender Lump Under Armpit Causing Localized Numbness Indicate?

I ve had a lump under my left armpit for a while now, and it s never caused me discomfort until recently, it s always been detectable when you move your fingers around the area but just recently it s been tender and now it s causing localized numbness. I went to my doctor who told me to get an ultrasound of the area which I m going to do, but for now I m waiting for my appointment. I just want to know if you have any idea what this may be before I get the ultrasound because I m getting scared. I m an 18 year old female and I ve been in good health for a good amount of time.