Brief Answer:
it's possible

Detailed Answer:
Hello,

although it is possible it's not very likely. Antibiotics may cause colitis and colitis may present with bleeding but other causes are more likely like an angiodysplasia, diverticulosis, ischemic colitis, bleeding hemorrhoids, etc. Antibiotics may also alter the normal flora of the colon and affect the bleeding time particularly to patients who use coumarinic anticoagulants (therefore making bleeding last for longer than usual).

Another important potential cause of major bleeding from the rectum includes upper gastrointestinal bleeding. This kind of bleeding is the most dangerous one, as it may rapidly cause fatal blood loss and all the associated symptoms (low blood pressure, fast heart rate, fainting, etc but upper gastrointestinal symptoms might be present as well like hematemesis, burning pain on the epigastrium, etc).

I hope you find my comments helpful!
Kind Regards!

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