drooping ear

One of my precious rabbits ear started drooping a couple days ago and over night there was a quarter size bald warm spot at the back base of his ear. Help, what is this and how to I help my rabbit?

Hello, I noticed yesterday that one of my bunnies ears is drooping down more than normal. Everything else with him at the moment is fine eating, pooping Etc and the ear itself does not have anything obviously wrong with it.The only thing that happened out of the ordinary was he had his nails...