I Have A Lump In My Right Armpit. I Am

I have a lump in my right armpit. I am 49, not sick, the pain is in my armpit thru to my shoulder blade. I now have trouble using my right arm for daily functions, due to the pain. Had blood work done, blood work came back great. What can this be??? It's been going on for over s month, and getting worse, did not injure myself, just starting to really worry now