The most likely diagnosis is ischemic colitis. The most common causes of acute, severe lower gastrointestinal (LGI) bleeding are colonic diverticula, angiodysplasia, colitis (due to inflammatory bowel disease, infection, ischemia, or radiation therapy), and colonic neoplasia. Other than colitis, LGI bleeding is typically painless. Ischemic colitis is due to a temporary interruption in mesenteric blood flow and typically occurs in older individuals with significant cardiac and peripheral vascular disease. Typical symptoms are the acute onset of mild, crampy abdominal pain with tenderness on examination over the affected region of the colon. Bleeding may occur early but often occurs within a few days of pain onset.

Angiodysplasia is most common among the elderly and usually presents as chronic or occult blood loss, but it can also cause acute painless, hemodynamically significant bleeding.

Colonic neoplasms may present with bleeding but it is typically of small volume and is not associated with abdominal pain.

Bleeding from a colonic diverticulum typically is acute and painless. Bleeding stops spontaneously in approximately 80% of patients but recurs in 10% to 40%.

Patients with ulcerative colitis almost always have a sense of bowel urgency due to rectal inflammation. Frequent watery bowel movements are typical, and bleeding occurs with more severe inflammation. The onset of ulcerative colitis is typically acute, and patients often remember when symptoms first started. Severe abdominal pain is an unusual manifestation of ulcerative colitis and suggests a complication such as toxic megacolon or perforation. Most patients with toxic megacolon related to ulcerative colitis have at least 1 week of bloody diarrhea symptoms.