Try pouring white vinegar on them, or taking a piece of gauze and soaking it in vinegar and wrapping it around the area in question, leave it on for about 10 min.. If the spots turn white then there is a possiblity that they are genital warts.
Not to scare you, but what you are describing sounds strikingly similar to genital warts... The bumps look more noticeable when you are erect because they are stretched out. You need to tell your doctor to take a biopsy of the area if he/she is uncertain what it is. Come back to the forum if it happens to be hpv, there are a lot of good people here.
My Best,
Rainn
Go see your doctor again if possible, if you are willing to get them biopsied ask him if he will, though you might not want to do that over nothing.