Hi, I Have A Tender Ligament Of Some Sort That

Hi, I have a tender ligament of some sort that stretches from my armpit into my bicep. I don't know that it is a ligament, but it is long and stretchy and seems to start in my armpit. I don't feel a similar 'ligament' in my other armpit. It is somewhat tender, and if I pull my arm in one direction it hurts slightly. I don't feel any discomfort unless I stretch my arm away from my body, but when I do I feel a tenderness that goes all the way down to my forearm.