I have noticed a lump within 7 year old Elvis's right nipple. It is the size of a very large pea and doesn't move around, as if it is attached to the skin. His nipple itself is now swollen and black and tender to touch which is how I have noticed it this morning. I am just trying to find a vet appointment now, does anyone have any idea what this may be? I have never seen anything like it. He is completely his usual self, eating everything, running around so seems very healthy.