My Mother Has Been Breaking Out In Hives And Itching

My mother has been breaking out in hives and itching and vomiting. I have tried claratin,zantac,zyrtec and famotidine at same day. Instead of getting better it stops for a couple of hours and starts back up again. She hasn't taken any new meds. Usually I have to take her to the emergency room to stop it. How do I find out what is causing this?