What Does Dent In The Back Of Head Mean?

I have a dent in the back upper area of my head. There is tenderness in the surrounding region that feels like a bruise. The dent hasn't been there all of my life, but maybe for the past few years. I recently bumped my forehead on my car but I only had a small knot in the front for about five days (that was about three weeks ago.) I don't know if that was what triggered the pain, but it seems as though it has become more sore since around that time. I don't really get headaches or anything but the tenderness is becoming more and more unpleasant. I have no idea what it could possibly be.