Its common to have weakness, dizziness
etc after recovery from a severe viral illness such as Flu.
Its better that she rests at home for a couple of days than to strain herself.
Give her plenty of fluids- water, fresh fruit juices, vegetable soups, porridge etc. Strict bed rest along with Oral Acetaminophen
in right dosage thrice a day- will help her.
How ever if head ache is severe, associated with vomiting, drowsiness
etc or she refuses to eat/ drink- you might have to visit your Pediatrician
Hope my answer helps you.
Wish your daughter a speedy recovery.