You are definately sure it is hives? You are probably right, just want to make sure. Finding the cause for hives is like asking yourself who gave me a cold? It could be an endless of things. In my medical book that I read too much,

it states that hives are generally brought on by an allergic reaction for it is the release of histamines in the body that cause it.
But it does say that certain viruses can cause hives such as hepatitius B and the Epstein Barr Virus. Is there an outbreak of mono in your neck of the woods?
It also states that out breaks of Candida Albicans and hives can go hand in hand. Oh, that's a shock, huh?!
I will list what the book says are some of the most common reasons for hive outbreaks:
Animals, dander and saliva
Aspirin
A gout medication that I don't want to spell out
Antimony, a metallic element present in various metal alloys
Barbituates
BHA and BHT
Bismuth, another metalic element present in metal alloys
Cancer, especially luekemia
Chloral hydrate, a sedative
Chlorpromazine, a tranquilizer
Cologne or perfume
Enviromental factors, heat, cold, water, sunlight
Eucalyptus
Exercise
Flourides
Food allergies
Food collorings and preservatives
Gold
Griseofulvin, and antifungal medication
Hyperthyroidism
Infections, strep, hepatitis, parasites
Insect bites
Insulin
Iodines
Liver Extract
Makeup
Menthol
Meprobamate, a tranquilizer
Mercury
Morphine
Opium
Para-aminosslicylic acid, an anti-inflammatory drug
Penicillin
Phenacetrin, an ingredient in some pain medications
Phenobabital, a sedative
Pilocarpine, a glaucoma medication
Plants
A polio vaccine
Potassium sulfocyanate, a preservative
Preservatives
Novocain, an anesthetic
Quinine
Reserpine, heart medication
Saccharin
Salicylates
Soaps
Shampoo
Sulfites
Tartrazine, a food dye and an ingredient in Alka-Seltzer
Thiamine hydrochloride, and ingredient in some cough medications
Here is some to get you started, HB. Hope you figure it out!!