could it be the muscle that controls your penis? If it's not any of what I asked then you might be safe, but if i were you I would arrange a time to go see a general doctor and get checked out specially if it gets bigger over the week.
Dave Gorman does the Homebase adverts. Shame really as he isn't at all suitable and I had a lot of respect for him until he did them. He was one of the last people I would heve expected to do an ad. We all need an income though so can't blame him.