If your dog is pooping blood it would be wise to take that animal to the vet immediately. It could be a lot of different things. From something torn down to a virus. One of the viruses is parvo which can and will kill a dog especially a puppy. There is 2 strains of parvo and even though one of them are more of a killer of puppies the 2nd strain can effect an otherwise healthy adult dog.. But if your dog is bleeding from it's rear end , the best bet is to take it to the vet immediately.