A sting would be my thought as well, but check her right ear, since it looks very swollen. Check for infection, drainage or gunk in the ear canal with a QTip. Does she have any signs of respiratory infection? I would give her 1/4 tablet of Benadryl 25 mg orally today to see if the swelling goes down. Look around the yard and coop for any bees, or possibly a hole in the ground where yellow jackets may be swarming.