28 mo old had bad rash that caused to hold her stool from pain & even after rash is gone still holding stool. She's dev'd fever twice. What's causing?

Reassurance. Reassurance and time will change things around. She's had a bad experience, and since the rash and the burning are no longer present, she will regain confidence again. Make sure you provide extra fiber (add fiber powder to her juice), and extra water, to make sure her stools are soft, and she does not develop constipation. The fever episodes are most likely unrelated.