Yup. Especially since she was supposedly dehydrated at the start.
My guess is that she wasn't dehydrated and the headaches were from, *ahem*, something else.
No, as I said it’s happened to me and the headaches are caused as the brain begins to swell. The salts balance in your body is very delicate but crucial. The sodium-potassium balance is the key.
A gallon of water in that period of time is probably too much for anybody. Electrolytes are crucial for hydration during hot weather exercise or any activity.