What Causes Gray Or Brown Spots On Legs?

I have some small, dark gray or brown spots on my legs that have appeared for a months. Not painful but there is a bump. Sometimes they come to a head and I can pull a waxy brown pit from them. This one is a little bigger than the others and yesterday when I tried to pull the pit from it or see if I could drain pus, nothing happened. Today, it is surrounded by a inch-wide circle of red and is very sore. What are these spots?