What causes medical bleeding from but?

Tissue injury: This is generally from tissue injury at some site. The bright red blood is generally from the anus/outlet from a fissure or tear. This can occur from various issues. If the blood is darker red to purple or black it would be from higher up. When you post, the more descriptive you are with symptoms the more likely we can help.

Rectal bleeding : This can come from hemorrhoids, diverticulitis, inflammatory bowel disease, vascular malformations. See a doctor