How Do I Know If My Child Has A Heat

how do i know if my child has a heat realted illness or the flu. We live in Iowa where there have been heat warnings. Our 8 year old was complaining that she didn't feel well and had a headache, we gave her tylonal and she got sick 15 minutes later. she has since 4 times in the 5 hours and sleeps in between bouts of getting sick. she has not gone to the bathroom and her skin feels clamy