First, make sure that there is no soap touching her vaginal area, of any kind. Shampoo, for example, after washing her hair may be getting trapped in her vaginal area and not being washed away. Also, the soap you use to clean her body (body wash, etc) may also be getting trapped there even if you are not applying directly to that area. I recommend that after you are totally done with bathtime, take a few seconds to run warm water over her vaginal area (ensuring you are getting in the folds) to really rinse off any soapy residue.
Also consider other irritants. How do you launder her undergarments? You may want to use a All-Free or Tide Free (of perfumes or coloring) or even Dreft for her underwear. Also, if you are using fabirc softener, I would stop that for a while to see if that helps.
In the meantime, I agree with the other responder that I think it is time to see your doctor again. Your daughter may have a yeast infection or another condition, lichen sclerosis, with occurs often in very young and very old females from vaginal dryness (not enough estrogen to produce natural lubricants). This can cause a lot of itching and burning.