A hot spot is a warm painful swollen patch of skin that loses hair and can have a bloody, pusy discharge. They can enlarge by inches within an hour or so. Batheing a dog and not rinsing well tend to preclude these at times so be sure after a bath you rinse your dog well.
To treat: rinse the area if you can with warm water and dry carefully or use baby wipes to clean it, then dust lightly with Gold Bond Medicated Powder.