Are you using a product or clothing, bed linens etc that might be causing this at night? Try to think if this could be the case- or any evening meds? Hives usually are a result of allergy.
The Hives Disappear Completely During The Day And Reappear Every Night. Do You Know Why?
It has to be something with the bedding that she get a allergic reaction from, you can have her tested for allergies to see what she is allergic to the best to you....