What Are Ingrown Hairs That Ooze Pus And Black Rock On Popping?

ive have a few ingrown hair looking masses that get about the size of a dime and when they are popped there is a black rock looking thing that comes out along with pus. ive had one on my back and two in my armpits. any idea what it is? the mass that comes out is usually around the size of a small pea