I noticed this yesterday afternoon. I brushed Rusty's teeth very early in the morning and didn't notice it then?? You can see the pink skin on the edge of his nose where it meets his mouth and on his mouth??
Bugs, who had very bad allergies, used to sometimes scratch her face until it swelled up and looked like that. But, it could be a reaction to insect bite or some sort of contact reaction, skin infection, or...? Let us know what your vet finds. Hope he's not too uncomfortable.
I talked to Rusty's breeder today and emailed her a photo of his boo boo. She said she thinks it might be a chemical reaction to something! We walk him 2 miles a day around a lake in our subdivision. The grass is well manicured and it does get chemically treated now & then. I think she may be onto something. I will have to be more careful about where Rusty sniffs. It doesn't seem to bother him or be spreading and Stickers is still fine so I'll keep an eye on it.
Otis used to get something like that also. My vet suggested we change his food dish to a stanless steel one. He had a hard plastic food dish, and apparently dogs sometimes get a reaction there from them. It did help. I don't know if it was coincidence or not, but it went away.