Wish I could help! My DS has this over and over. His allergists have no idea why. It is very frustrating for us. He is also on 3 different daily antihistamines though.
Has she had allergy testing? Dust, for instance, is everywhere. Have you checked the pollen levels? They could have been high those days. Is it possible that she is newly allergic to a food. When your body has had enough you can develop allergic reactions to foods you have had many times before without a problem. Here is my favorite fall-back, many viruses can cause hives and if her body was fighting off a virus she could have had hives as a result.
Sorry, I know it sucks and your dd is probably miserable! Baking soda baths can help during times when hives are only slowly receding. If your doctor hasn't explored all options I would probably have her assessed though as it can be a sign that she is developing an allergy to something new. (Or to her first something as I see since I read the original post there were more posts and you said she doesn't have known allergies.) :grouphug: