What Could Black Bumps In Arm Suggest?

Hi. I'm 15 years old. Recently I developed a black bump on my arm. I was scratching at the bump today, and the skin broke. I could feel something hard just beneath the skin, so I decided to to try to get it out. I extracted a small pebble-like stone, about 2mm across. It's about the shape of a bean. This has happened before--about a year ago. Now that it's happened a 2nd time, I'm a bit worried. Any ideas to what this is? Should I be worried?