I had the same thing off and on for years. I always assumed it was an old injury.
2 years ago, it got worse to the point that I was in pain even standing. After just 2 days, I noticed a large red lump. It turned out to be a Pilonidol Cyst, and had to be surgically removed. It is apparantly very common, and not much to worry about. I would go to your doctor or even a walk in clinic, and ask them to check.