G.T.
You might try giving her a little bit of a fleet enema to soften the hard plug she has stuck there. The miralax will soften up some of what is still in her colon trying to come out, but I assume she has a hard rock first that needs to be removed. If she goes to the ER chances are that's what they will do, they will give her an enema. Just keep giving her fiber and a lot of water to drink. Dont forget that pain meds, even childrens advil, tylenol and such help to cause constipation....
If you give her enough fiber with water and miralax, she'll eventually poop up a storm. Put some vaseline in and around her anal area, it's probably sore from some sharp, hard poopy.