It looks like de pigmentation. Maybe you were having some thyroid issues at the time or had some sort of an allergic reaction. I have a spot of my vagina too. I also have hashimotos which is an autoimmune condition. I've found that drinking plenty of water and getting good nutrition is helping me feel so much better overall. I try to avoid too much caffeine, red meat, spicy foods and gluten. Although i still have gluten i limit it a little more these days.

Spreading varies from person to person. Mine continues to spread over the past 50 years. My siblings have just a couple very small spots but theirs has never spread.