It sounds like vitiligo, but why would your doctor not have said so if that is what it is? Vitiligo is harmless, and not uncommon, some famous people have had it (I believe Michael Jackson was said to have had it, and there is a well-known model who has it). But no doctor would shrug it off as a birthmark, they would call it by its proper name. You could google it and see if it looks familiar.