I would call a vet, it could be a number of things including a bee sting or similar, an abscessed tooth, something in or near his sinuses etc, though if it was tooth related I would expect him to be a little more reluctant to eat.
Is it hard or soft to the touch? Any heat? Is it solidly stuck on place or able to be moved? Does he otherwise seem normal? No temperature or change in breathing, hasn't become more clingy or harder to catch than normal? I've got no idea what this could be to be honest, but all the above could help to figure out what it is, along with the advice of a good vet, especially since after 3 days it hasn't continued to decrease in size
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms