As
@Roger rabbit says, you ought to have had stool sample taken.
The
most probable reason for the bleeding it the stress on bowels by diarrhea - the contents of your alimentary tract fly through you so fast that the stomach acid doesn't neutralize properly and wrecks havoc on your intestinal epithelium. Either diffusely, or you have duodenal ulcer.
Imagine blowing your nose to often - you end up with bleeding eventually. (now imagine blowing it with acid).
You might also have some mild intestinal infection which the antibiotics will help with.
Now, if the blood was really really almost black, then the most probable reason would be
stomach ulcer.
None of these are extremely serious and will go away as your stomach settles and/or ATB start working.
Now, had the sample been taken (just a swab is enough, really) after a first few days there would be results available - if there's Clostridium Difficile (nasty little fucker) there it would require a change in therapy. If there's not, you are most probably okay.
Fun fact: The best therapy for rampant C. Difficile infection is fecal transplant from your family members. Yes, exactly what it sounds like.
Now, if even after a long time the bleeding doesn't stop, you might want to be checked for ulcers and Helicobacter Pylori, colorectal cancer or autoimmune diseases of bowels.
You said the blood wasn't bright red, so I don't think it's hemorrhoids.