I have what would best be described as a fluid filled bubble of skin on the underside of the head of my penis (around 1/8 - 1/4 in. in diameter and height). I have looked all over trying to find out what it might be but all I seem to be able to find is info on pearly papules, and it looks nothing like the images of those I've seen. I am circumcised in case that's relevant. I'm also very confident that this is not an STI. I've had it for as long as I can remember (before I was even masturbating if I'm remembering correctly).