What Causes Hives On Body And Uncontrollable Itching?

Hello; My mother is 70 years old and about a week ago she got hives all over her body, uncontrollable itching. She was put on Benadryl and Prednisone which only helps for 8 hours. Now she has stomach cramps, nausea. She has seen three doctors two said its food allergies and one medications. She has changed her diet to only rice and white bread and not taking her cholesterol med. Please help