My dog gets red patches during the winter. I learned that it's caused by dry skin. The skin itches, and he scratches and scratches. Then hair falls out from scratching, he gets swollen and bleeds.
This may or may not be what's affecting your dog, but it was for mine. I bought some dog skin cream which helped ease the pain. I also began adding vegetable oil (or sunflower oil) to his food, which helped also.



