It might be the worms. Severe worm infection can also cause bloody stool. Give your dog a light diet. Kindly take the dog to the doctor as soon as possible. Also see the link below to look for parvoviral infection and symptoms of worms. Here you go and hope your dog gets well soon.
www.workingdogs.com
www.cah.com
www.workingdogs.com
www.cah.com