I've had a bump on the side of my shaft for over a year now. Last month I had a blood and pee std test and everything came back negative, though I do not believe those tests include hpv. It hasn't spread, but is on a cycle of growing (and then shrinking), and I pop it and a little pus comes out which shrinks it down but even after popping the bump doesn't feel empty. What could this be? Thank you in advance for your help.