Lump and pain in help?

Ive been dealing with this for 2 months now. It is not mastitis, a cyst, or shingles. It is about the size of a walnut and on the side of my by my armpit. My swells if i do too much and it is extremely painful. There is no rash or skin change at all. They have done sonograms and x rays and have not found anything unusual. i have been on antiviral and anti biotic medications for this. Nothing is helping. any ideas?