M.P.
Hives are an allergic reaction. That's why the benedryl helps. She has probably developed an allergy to something over time. Doesn't have to be anything new. The allergy could be environmental or food.
I suggest a visit to a pediatric allergist.
I have food allergies. I can eat a certain amount of the food before I get an actual allergic reaction. I suggest you start by giving her just a few specific foods that are easily digested and see if that helps. If it doesn't remove one food at a time over several days.
Common allergens are soy, tree nuts, peanuts, milk protein, eggs. I would also eliminate raw foods. Cooking changes the protein in foods so that they are often tolerated when raw foods aren't.
You could use a hypoallergenic detergent to see if that helps.
However, if the hives continue I'd take her to see a pediatric allergist. It may take a week or two to get in; so I'd call to make an appointment when the office opens on Tuesday.