Is he in pain or showing any signs at all of discomfort? Is he eating? It looks like it could be some sort of bite, does he go after bees at all? My girl Tess loves to try and catch bees and I'm always worried that she's going to get stung.
If you are just noticing it today, chances are that it just happened then. Something like that, you just don't over look, unless you haven't been around him due to work or something else.