Get her mouth checked for ulcers.
It might be Hand Foot and Mouth disease, that is caused by a virus, presenting with red rashes more in palm and feet, ulcers in the mouth. Because of ulcers, child will not be able to eat well. And its common in this age group - below 5 yrs.
Its a self limiting one. Just give her Acetaminophen
in a dose of 15mg /kg/ dose repeated every 6 hrs. See to that her fluid intake is okay.
If you are very worried, have a complete blood count
done that will give you information about the white cell counts and platelets too.
If viral, you need not worry. She will be fine soon.
Hope you find my answer helpful.
Wish a speedy recovery for your kid.