Hard bumps on muzzle that come and go?

[JLP]

My 8 year old rough collie has been getting hard bumps on his muzzle that come and go. He's had this twice now. They don't last long at all. It sort of goes like this:
Day 1: Small bumps, hardly noticeable.
Day 2: Medium bumps, noticeable.
Day 3: Very large bums, very noticeable.
Day 4: Completely gone.

By the time we call the vet and get an appointment, the bumps are gone.

The bumps are under the skin, are hard but can be moved a little, don't discolour the skin and don't hurt (as far as I can tell).

At first I was thinking allergic reaction, but the bumps don't seem to be itchy and aren't red. He has also been on the same food for the past year and the same brand as long as I've had him. He's eating 5 1/2 cups of Innova Large-Breed Senior and the only treats he gets are from Blue Buffalo. I haven't introduced any new treats.

Anyone ever had to deal with this or know what it might be? The vet doesn't know because the bumps are always gone by the time we can get an appointment.