Thank you for the query.
Such pain while sitting can be caused also by pilonidal cyst
localized on the area where the tailbone is. This condition is caused by hair ingrowing which irritates surrounding tissues causing pain. So maybe this is what brings pain. Such cyst can be visible in between the buttocks as some kind of small hole (usually there are two holes). If there is any, you should visit a surgeon as the only treatment is removal of it.
can be full of stool especially if the test were performed before the bowel movement. This is how it works. So if your daughter does not complain from constipations, this is quite normal thing.
Hope this will help.