What Does A Lump On The Back Indicate?

I have had a small, round, hard bump on my back toward the right side that used to be skin colored but now has some pigment on it and seems to be larger than the last time I looked at it in the mirror. I have had a dermatologist do a full body check of my skin and she said she didn t know what it was. Wasn t concerned. It has clear borders and is symmetrical. What bothers me about it is that if I happen to lean against a hard surface, like the back of a wooden chair, and it presses on this growth it s extremely painful. Could there be extra nerve endings in this thing or what? Should I have it removed?