why?

Question
why does my dog poop out blood ?

Answer
Your dog can have blood in its stool for many reasons. It could have parasites like whipworm, a bacterial infection such as giardia, colitis, constipation, a foreign body it is trying to pass, irritable bowel symptom, and on and on.... all the way to cancer.

Take a stool sample and your dog into your vet for a diagnosis, and he/she will most likely give you some meds to clear up the problem, assuming it is one of the easier ones.