Agreed with the above about seeing a vet ASAP and trying a gentle shampoo or dye-free, fragrance-free soap like you would use on a Baby. With it being near her vaginal area is it possible she is squatting so much to pee in the mornings she is getting dew there and picking up pesticides or other things from wet grass in your area? I would wash her, very carefully so as not to make her raw, after each potty break. But with the pinkness in the other areas, it sounds like more than just that.
In my experience, Pits are notorious for skin allergies. Allergies to foods especially. (I haven’t read all the posts above and they may address this too) And that generally shows up all over. The white Pits usually have the pinkness on their chest and in larger areas than you are describing but this could be the beginning. Watch it carefully and do get to the vet. She could get really uncomfortable if you let it go too long.
