This is the original expert.
Clearly a posterior bleed was not missed, since you had it surgically repaired.
And, yes, after two days of massive bleeding, it would be very common to have black stool.
The surgeons would have estimated your blood loss. As long as the repair was successful, there is no reason to suspect continued bleeding.
There is no reason from your story to suspect any other source of bleeding.
Typically, the hemoglobin measurements are accurate 1 to 2 days after the bleeding is stopped.
If you had a lot of blood loss, and particularly if you have any symptoms, such as fatigue, and you did not have hemoglobin monitoring, that would be good to do today. Symptoms would dictate going in.
We don't know when this all occurred. If it all happened a months ago, obviously, there is no current concern.
If you have already had postoperative follow up, with no more black stool, clearly you are OK.
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