What Causes A Lump On The Shaft Of The Penis?

I have noticed a lump on the shaft of my penis that is not on the top of my skin but seems to be under it. It moves with the skin and is about as small as half a grain of rice. I was masturbating when I felt it under my hand. During erection it seemed to get bigger. After it dropped back down to the size of half a grain of rice. Now 24 hours since I have noticed it, It has not got any bigger. There is nothing showing on my skin. No discoloration and does not burn, itch, or anything. It feels hard enough to know its there. I have hair around the area but don't quite know if it is an ingrown hair. I have no STDs since I have not been with anyone in 4 years and was tested for STDs 3 years ago. What could this be