Hi, I Have A Hard Lump Toward The Back Of

Hi, I have a hard lump toward the back of my armpit, almost to my back. Could this be a lymph node or is it too far back...where does your arm pit end? Lol. I am going to gp tomorrow, my mind is racing. My mother had breast cancer. I've had this lump for several weeks, it is of significant size, no pain, I feel like I can move it around. I am definitely more tired than usual the last few months. Thanks.