needs to be evaluated with a colonoscopy
(camera through the anus) to make sure your grand mother does not have any colon cancer
or large colon polyp. The most common cause of bleeding during bowel movement is hemorrhoids, however the only way to have the most certainty is with the colonoscopy. Other alternatives test exist such as Computed tomography
colonoscopy, barium enema
, etc, but are not as good, and you cannot do anything about it, so again I recommend a colonoscopy.
The fact that she feels weak, may indicate that she is anemic due to bleeding during bowel movements.
Hope that helps. Let me know if you have any further questions.