It sounds similar to something my non-CAH daughter used to get.  She would itch and scratch until it became sensitive to the touch.  When she urinated it caused some minor pain and would wine until we attended to it.  (Although I’m not quite sure there was a lot of pain during a bowel movement)

The way we treated it was to put a cream form of Cortisone all around on the labia and if necessary, the anus.  If you try this be sure to treat the anus last to insure cleanliness in the vaginal area.

Have they tested her for a yeast infection?  Just a thought.

I wish you and your daughter the best of luck in unlocking this mystery.

Let us know how she does.