Occasionally I will get a small dark reddish bump (smaller than a pea) below my armpit, and it looks to be filled with blood. It is mildly uncomfortable, so I usually end up squeezing it and some blood comes out, but it leaves a small hole and still looks dark inside. Eventually the hole mostly closes up but leaves a scar and the texture of the skin over that area is left a bit softer and kind of wrinkly. I have had several of these, always under the left arm, near my breast. I know it is not a pimple or ingrown hair. Any idea what this could be and what s causing it?