Hi and thanks for using the forum.
What you have described does sound like vitilogo. However, since it has not spread or changed, it does not seem to be that.
I think your doctor is right in that as long as it doesn't change, I would not worry about it. If you are really concerned, you could ask you doctor to be tested for an STD, but again, I think that is probably just an unexplained loss of pigment. I don't believe that there is anything that can be done for the skin once it has lost its pigment. Good Luck,
Bridget
Add:
The doctor couldn't explain it and said of it "It just looks like a loss of pigment, if it changes at all, come back and see me."