Final answer:
The most likely cause of bleeding is antibiotic-associated diarrhea (AAD) caused by Clostridium difficile (C. difficile).
Step-by-step explanation:
The most likely cause of the bleeding in this case is antibiotic-associated diarrhea (AAD). AAD refers to diarrhea that develops as a result of disruption to the normal microbiota due to antibiotic treatment, and Clostridium difficile (C. difficile) is a common causative agent. Antibiotics can disturb the balance of bacteria in the gut, allowing C. difficile to overgrow and cause symptoms such as diarrhea and abdominal pain. In severe cases, AAD can lead to bloody diarrhea and other complications.