Pepto bismol can cause internal bleeding; it's similar to aspirin.
Immodium is downright dangerous to dogs, in GSDs you don't have to worry about the MDR1 gene, but it can cause the GI tract to shut down. Stop working, this is quite dangerous. It can cause a pet to bloat, even.
Treating Diarrhea in Dogs with Imodium - VetInfo
Not to mention, when anyone/anything has diarrhea, it is the body's way of ridding itself of an irritant.
Rather than shut the GI tract down, it's best to figure out why the diarrhea is happening.
With bloody diarrhea, it is called "colitis", but it is secondary to something that's causing the irritation. This could be a mild thing, something the dog ate, or severe, such as "HGE", or pancreatitis.
It's really critical, after even 1 day of bloody diarrhea, to get the dog to the vet, and not try "home remedies" which may not work and may even be dangerous.
The best treatment, with your vet's approval, of non-specific colitis, or non-specific diarrhea, with or without blood, is either Tylan or Flagyl (metronidazole). But you need to check with your vet about these drugs as well and get correct dosages.
And above all, find the underlying issue and address it.