What Causes A Small Hard Lump Inside The Glans Of The Penis?

Hi there, I am a 27 yr old male, (5ft 8in and 11 stone, smoker but not a drinker, no pevious problems in this area) and haven't been sexually active for about a year. I have recently noticed a small, hard lump inside the glans of my penis, just to the right of my urethra. It doesn't hurt unless I squeeze it a bit, but it does get larger or smaller depending on arousal, and remains solid for a couple of hours after ejaculation. It is about the size of a ballbearing when I have an erection and shrinks to about half size when my penis is flaccid. Any ideas as to what it could be? Thanks