Bulldogs World

My 2 year old bulldog has never had hives before, but the last 3 weeks they have been coming and going. We live in the Chicago area so initially I thought maybe it was the seasons changing and something was pollinating. I have been walking him on a leash so he can't get into any weird plants/grasses that maybe were setting it off. It seemed okay for a week and again last night it came out of nowhere he hadn't been outside recently. I feed him orijen red meat dry food and raw food. He has been eating raw and orijen his whole life so I wouldn't think it is food related. Could he randomly come up with a food sensitivity that would cause hives? He doesn't eat any grain or chicken even with his treats or chews. His hives are limited to his legs/sides when they appear, and he doesn't itch at all and has no idea they are even there. The vet said to give benedryl when they show up unless it gets serious and they are on his face or he is breathing weird. They usually go away on their own within an hour, or after 1 benedrly and he weighs 70lbs. No change in shampoos, housing, or any cleaning supplies. He walks in the same places and today he is fine no hives so I am very confused! Has anyone else experienced this before? Any suggestions would be greatly appreciated! It doesn't bother him at all but it bothers me not knowing what is causing this. Thank you!