My daughter has a rash, the runs, and will not eat. What should I do?

See your doctor. Keep her drinking (rehydration solutions available at pharmacies are best) while waiting for the doctor to call back.
Viral illness. Likely it is a viral gastroenteritis (viral GI bug) which is causing diarrhea and viral exanthem. Please focus on hydration and see your physician.
Viral infection. To start plenty of liquids and if she looks sick call your doctor.