What would cause this redness / swelling on my guy’s muzzle?

Woke up at 5 this morning to my dog whining in his crate. He had pooped in his crate and was absolutely beside himself. Totally unlike him. I also noticed his muzzle was red and puffy, plus he was licking his lips a lot. I gave him a few Benadryl as my first thought was an allergic reaction to something. I bought him into bed with me and he settled down and went right to sleep. It's now been a few hours and his muzzle still looks sore. I have a vet appointment for him on Monday, but I'm wondering what I can do to soothe him in the meantime. Obviously if it gets worse, l'll take him right into emerg. There's been no new food products introduced lately. My only thought is maybe he got into something In the backyard?