Horizontal pimple?

Patient: A few weeks ago I found a mark on my inner thigh that to me looked like a dark stretch mark. It doesn’t hurt and was only itchy once when I first noticed it. Recently I looked at it again and it turned into what seemed to be a pimple. But instead of round it was laying raised horizontally. At one end it had a little black spot. When I went to pop it, the “white head” was very solid and long. And as I squeezed, it got darker (almost like a blood blister). But it did not bleed and did not hurt. What could it be?