Katie had one last summer on her muzzle... similar spot.... Got big and hard like a golf ball. When it came to a head finally, I gave it a squeeze and it drained.
Poor girl was sore, not sure how she grazed with it so tender.. I assume she got a splinter or maybe even stung by something that started it festering..
Anyhow..she is no worse for wear today... she is all cute again... ;)