Just keep a log of food/meds/etc for the next few months..
It took me almost 3 months to figure out I got the Hives every time I took ibuprofen... which I was taking in order to make the hives go away
Now I only take a Acetaminophen or Aspirin if I ever need it.
Make sure she wears "loose" clothing when she has the hives as any place that rubs (like a waistband of pants) can in turn create more hives to be itchy and uncomfortable with.
It took me almost 3 months to figure out I got the Hives every time I took ibuprofen... which I was taking in order to make the hives go away

Now I only take a Acetaminophen or Aspirin if I ever need it.
Make sure she wears "loose" clothing when she has the hives as any place that rubs (like a waistband of pants) can in turn create more hives to be itchy and uncomfortable with.