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Blood in the fecal matter is not always a cause for alarm. It could indicate irritation to the GI tract. This can be from a viral illness, something that the dog ate, or even a change in treats or food. I would go ahead and collect a stool sample (the bloody one) and see the vet with it (most will do this without an appointment) to rule out parasites as the cause. This is the number one cause. Then I would go from there.
This does not mean that there is a bone that has abraded the intestinal tract or anything of the sort.
Let me know if you still have questions.