Looks like a bee sting, problem with bee stings is that after the pains wains, the sting itches like a b*****d for 3 days, first day it hurts, second it swells. Its the scratching that takes the skin off due to the itch. Try dabbing it with salty water 4 times a day and see how you go. Not saying this is definitely the cause, but my GSD eats bees like cornflakes, and every now and then he meets his match. ?