The GI mucosa is highly vascular and it is not uncommon to have a small degree of bleeding from it. I'm not sure what starts it, some sharp roughage that didn't get fully digested? stretching? I don't know. The same thing commonly happens in people where they will occasionally see a streak of blood in the feces. When the blood is in discrete pockets or streaks, rather than well mixed with the other feces, I take that to indicate it happened towards the end of the digestive tract. If the bleed was farther up in the small intestine, it would be mixed with the feces more homogenously. Also, I'm guessing the fibrous material around it is mucous being produced by the rectum. It can collect between fecal balls, I'm guessing he is seeing a mass of mucous and blood that collected between two segments of feces while in the rectum.
When you hear about a calf with blood in the feces, the obvious impulse is to say cocci. But this doesn't sound quite like a cocci calf. Cocci is more often seen in calves younger than that, either the 1-2 month age range or around weaning time. Older calves can and do get it, but less commonly have it severe enough to show symptoms. Additionally there is no sign of diarrhea. And I think cocci calves will have the blood more thoroughly mixed in with the feces. And she is out on pasture where transmission is less common. None of these are guarantees it isn't cocci, but I don't think it is likely. There shouldn't be blood in the feces from a heat cycle either, that should all be vaginal. Theoretically maybe she could of defecated and then leaked some vaginal blood on top of the pile, but this is probably blood from the GI tract.
I wouldn't treat with corid at this point either. If anyone else in the group shows signs that are at all convincing they have cocci, or a fecal is conducted showing decent levels of cocci, then I would medicate the group with corid in the water. But if it is just this one animal with these mild symptoms, I would bet some sustain III pills would clear it up and you wouldn't have to medicate the whole group or repeatedly get this one in and drench her when cocci may not be all that likely in the first place.