I had something similar back in February. It never happened before, and honestly I thought it was like a zit or something caused from exercising and maybe my underwear kept rubbing it wrong.

I didn't go to that doctor to have it checked. In a week it went away. I was very nervous about the whole matter. My boyfriend and I are not sexually active and I read that it showed signs of an STD but I knew it couldn't be that...Anyways, I made sure to wash it and keep it clean. It ended up being a large blister (like I thought it was, my underwear rubbing it too much)...I was very painful and I could feel it when I walked, sat or stood a certain way...

The bump did turn white before the blister busted. When it busted it was all water.

If this sounds similar to what you are going through, I hope it helps...