So, like the title says, I have a lump beneath the skin of a stretch mark. I have some on the inside/back of my upper arm and that's where it is, technically the armpit area, but as far as you can get from the actual pit to still be classified as an armpit. The lump is maybe 1cm, purplish-red, hard, doesn't really move when I roll it around, and feels slightly sharp when I pinch it. It only hurts when I pinch it to try to move it, and it isn't warm or itchy. There is an enlarged pore on one side of it that seems to be where it originated from. I have a problem with excessive sweat, and it is an area of high friction (not due to restrictive clothes). I've done some light googling about what this might be, and I think it's probably a sebaceous cyst. I just wanted to get a second opinion and ask what I should do about it if anything, if it really is a sebacous cyst, all that.