Hives indicate an allergic reaction to something in your dog's environment. It is difficult to pinpoint the exact cause unless you see the hives appearing after a specific exposure. See your vet. Benadryl is often given for immediate relief of itching hives and steroids may be necessary depending on severity.
This is an allergy reaction, you can try to deduce it on your own, but I certainly don't suggest that, as allergies can compound and be fatal. I really advise that you check with your vet.
