What Causes A Painful Lump Under The Armpit?

Good evening. I am concerned about a lump under my left armpit. It is painful and a strange shape almost lobular. I was do with breast cancer in 2012 and underwent a bilateral mastectomy with the right breast being radical due to type 2b breast cancer with involvement of a lymph node. I am concerned that I'm paranoid. My oncologist said he can not feel the lump but I can. Are my fears founded?