My six year old daughter has a rash that started on her legs and now it is all on her back, underarms and her arms I gave her 2 teaspoons of allergy relief

Rash. Might be hives which are red, raised and intensely itchy mosquito-like bumps. Using topical anti-itch creams and an oral non-sedating antihistamine might help. I would also call her pediatricians office and speak with the on-call pediatrician.
See a doctor soon. Rash starting on legs could be something more than allergy. Does it hurt? Does she have fever? Does her belly hurt? Is it raised from skin or flat with skin? No way to tell without seeing the rash. Go see your doctor today. If it is 'hives' or welts, then probably not much to worry about. But can't tell without seeing the rash.