What could be causing my dog's rashes at night?

In about a month now, almost every night before he goes to bed, my dog's skin gets really red. The vet prescribed benadryl but it makes him drowsy that he doesn't want to get up, so disoriented the next day. I only give him 12.5 mg. This week, I eliminated his peanut butter, kept him away from the grass, even switched to grain free food. What could be causing this at night? Please help me. He will see a skin specialist next week but I am just so curious on why this is happening. Thank you for your help.

If you feed him at regular times he could be allergic to something in the food. It may be grain but he could also be allergic to something else in the food.