<s>[INST] I have a lump on my upper arm just underneath my shoulder that is about 2 or 3 inches big. It reappears every couple of days or weeks and sometimes it is really tender and other times I barely notice it apart from when I feel it is there. What could it be? [/INST] HiWelcome to hcmThis can be lymph node. You should see a surgeon for confirmation and mostly it will require biopsy.Regards </s>