What Causes Bump On The Penile Shaft?

Hi I have found a hard bump on the shaft of my penis. It is very tiny about the size of a very small pebble. It very easy to tell when the shaft is erect but with some searching I can find the bump when it is flaccid. It located about an inch below the head and is definitely under the skin. I very confident hat itâ??s not an ingrown hair or the like. I have been with my wife for almost nine years. I have had this bump for about a year, sometimes I think it has disappeared but it seems to come back. Could you provide some insight as to what this could be?