Hello, I have a lump underneath the skin in the antecubital region of my left arm. Not sure how long it has been there or how it got there, but I found it after trying to practice blood pressure in class. It is not painful, I can just feel a little pressure when I push down on the lump. I have not had any recent blood work done, and it can only be felt if my arm is extended. what could this be? thank you