Most likely bleeding is from a minor hemorrhage in the intestinal tract from who knows what.
Coccidiosis is a possibility, but I would expect diarrhea too rather than just blood alone.
Outside chance she is a salmonella carrier. Clinical signs of fever and diarrhea usually in calves under 12 weeks of age.
Carriers don't show clinical signs, but might have blood in stool for 2 or 3 days when under stress.
Salmonella can cause abortions, and carriers spread it in their manure aided by mice, rats, pigeons ect.
I'm not saying it's salmonella, more common in dairy calves, just thought I'd give you something to worry about. :tiphat:
If she continues to have blood in her stool, I'd treat with penicillin for 3-4 days.