It's most likely a cyst, or perhaps a collection of fatty tissue. Our bodies collect these things over the years, and we only notice them if our hand wanders over the area. I advise you not to worry, but to show a doctor so you'll know for sure. The only treatment for such things is to cut them out, but that's hardly ever worthwhile--the scar is worse than the bump.
Take care.
Dr. Rockoff
eerr... typo meant to say "Not to sure where this goes so i out it under Misc"