Once you get her cleared out are you on a maintenance to keep her moving?
She won't be able to hold it if you load her diet with fiber drinks or kombucha tea (organic section of your grocery - 1/4 cup for a couple days) or castor oil in a drink.
Maybe you could also ask the pediatrician about...
Have you tried giving her fiber - like metamucil and other brands?
You can give her castor oil.
Also a small cup of milk with a good bit of kayro syrup will make her go.
I feel for you and her. Been there. Sat many many days on the edge of tub and held my DD's hands and talked her through it...