If this was a sudden onset and you have ruled out access to any toxic substances, eating bark or twigs, and yet the vet still has not made a diagnosis based on tests that have been done....if it were me, I'd go to another vet. I had a similar incident when Tebow was 4 months old, new to the home, bright bloody stools. The first vet didn't seem concerned and it had been going on for 4 days. I took him to another vet. The parasite tests were negative & the meds she prescribed didn't work. A vet tech friend suggested another med, Albon (500 mg).... The vet gave me 5 pills ($19.25) and in 24 hrs, no more blood, no more diarrhea. Shame that it took a vet tech to have the solution.... Before I got that medication, I was out almost $500. And if I hadn't pushed for that alternative med, I might have been without my puppy.