Why is your dog peeing blood if she is spayed?

My first thought would be an incomplete spay surgery in which part of an ovary was left within the dog's body. Even a small ovarian remnant is enough to produce the hormones resulting in signs of heat, including swelling of the vulva and estrus bleeding or spotting. However, your dog will still be incapable of becoming pregnant, as the uterus would have been removed completely.

There are other conditions that can cause similar signs, including adrenal tumors and brain tumors, but an incomplete spay would be my primary differential diagnosis, particularly in a young dog that has just been spayed.