I had the major bleeding problem a few times before, but never this severe. This was the first time I went to the hospital because of it. The doctors did not identify the location of the former bleeding, but a future colonoscopy (after a more complete cleanse of my digestive tract) might reveal where the bleeding came from. The colonoscopy I had revealled that I have diverticulosis. That health condition consists of pockets called diverticula which form in the walls of the digestive tract (see https://www.webmd.com/digestive-disorders/what-is-diverticulosis and https://www.aafp.org/pubs/afp/issues/2009/1101/p977.html ). I was told I have numerous such pockets. Sometimes an artery (near or in a pocket) can break and bleed into the colon. The doctors told me that the gastroenterologist suspects that is what caused my bleeding, but the view of the inside of right side of my intestines was partially blocked (due to my intestines not being flushed out well enough).
The health problem might happen to me again and a doctor told that it happens again I might need a transfusion again. https://www.webmd.com/digestive-disorders/what-is-diverticulosis says the medical field does not know for sure what causes diverticulosis. That web page also says the following.
"Possible Complications
...
You can get a diverticular hemorrhage. This is rare. It happens when your arteries wear through the intestinal wall. It causes massive bleeding and requires hospitalization and blood transfusions."
https://www.aafp.org/pubs/afp/issues/2009/1101/p977.html says the following. "Diverticular bleeding is the source of 17 to 40 percent of lower gastrointestinal (GI) hemorrhage in adults, making it the most common cause of lower GI bleeding.1 In one study of 1,593 patients with diverticulosis, severe life-threatening diverticular hemorrhage occurred in 3.1 percent of patients.2 Most diverticular bleeding is self-limited, although it should be suspected in patients with massive and painless rectal hemorrhage."