What Causes An Inflamed Bump With Hard Dark Inside?

My daughter has a lump on her back of about 5mm, she thinks she has had it for about three weeks. It is dark coloured and looked imflamed. I squeezed it, and a black hard round stone like lump popped out of it. The stone like lump measures about 3mm diameter. The remaining bump on her skin is still slightly hard. Have you any idea what could cause this stone like lump?