Hi XXXXXXXXXX~
The most common cause of bloody stool in puppies is parasites.
All pups are born with parasites, and usually a good deworming will get rid of the problem, but since your pup came from a shelter, I wouldn't be surprised in the least if that's what's going on here.
My suggestion is to take a fecal sample in to your regular vet and have them screen it for parasites. If it comes up positive for worms, your vet will be able to help by sending out the right type of anti-parasitic medication to clear the problem up.
Parasites would also explain the pup being thin, having a 'gurgling' tummy and frequent diarrhea, so it's a good idea to have the pup's fecal tested as soon as possible, as untreated parasites can lead to death.
I hope this helps.
Certified Veterinary Technician
Associates Degree in Veterinary Technology. State and Nationally certified.