If it's hard and has been there for a while, you could have a keloid scar, which are pretty common. I have one on my left shoulder, as well. There is no way to really tell one way or another what it is without having a doctor look at it.
If it is a keloid scar, it can't be removed because the area would scar much worse. They can inject it with steroids to make it smaller. It didn't work for me, but it does for some people.
Still, you should never ignore any bump or anything unusual. Go ask the doctor. There is no harm in getting it looked at.
One question: Is it bothering you because it hurts or is it because it looks bad? (Personally, mine looks like a bullet wound.)