I am sorry to hear about this concern.
My primary concerns for diarrhea with or without blood include:
- Dietary indiscretion (eating something you do not know about)
- Stomach or intestinal foreign body
- Something that was given to them (new food, new treat, human food)
- Intestinal parasitism (not just the worms, but the microscopic bugs like Giardia and Coccidia). They can cause vomiting also.
- Toxin exposure would be less likely, unless you have reason to suspect exposure to a chemical.
- Less commonly, endocrine diseases.
- Generally, in older pets, I am also concerned about organ disease (liver, kidneys) and cancer.
The reddish color suggests some colitis (inflammation in the colon) is present as well.
So, tough to say, with only one episode, if this is going to be a serious persistent issue or is due to maybe a dietary indiscretion.
A vet visit would likely result in some medication for the intestinal tract, and maybe doing a fecal test to check for any intestinal parasites.
It is okay to give teaspoon or two of some pepto bismol, but I would only do it once and follow up with your veterinarian if diarrhea persists, or you observe any other symptoms like reduced appetite or vomiting.
Hope that information helps