I keep getting these weird black pebble looking things under my skin.?

This happened on my stomach, back and neck all at different times. It would start like a little zit or something, more like a little bump, but it wouldnt go away. Over time if i looked closely i could see something black in the middle. I would have to squeeze VERY hard to get it out. and it would just be like a very tiny black pebble looking thing, and it was hard. after it came out, the bump would go away. it would take months for the squeezing to finally get it out too. What could this be?