Diverticulitis: This condition is characterized by inflammation of the diverticula in the colon, which can cause abdominal pain, typically in the lower left quadrant (LLQ), and rectal bleeding. The symptoms presented match closely with diverticulitis, especially given the location of the pain and the presence of gross rectal bleeding.

Diverticulosis with Bleeding: Similar to diverticulitis but without the inflammation. The bleeding can be significant and is a common cause of lower gastrointestinal bleeding in older adults.
Colorectal Cancer: Although less common in younger populations, colorectal cancer can present with abdominal pain and rectal bleeding. The pain might not always be localized to the LLQ but can be a consideration, especially with significant weight loss or changes in bowel habits.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause abdominal pain and rectal bleeding. The pain can be more diffuse or localized depending on the area of the colon involved.

Mesenteric Ischemia: A potentially life-threatening condition where there's a lack of blood flow to the intestines. It can present with severe abdominal pain out of proportion to the physical examination findings and might be accompanied by rectal bleeding. Early diagnosis is crucial.
Intestinal Obstruction: While more commonly presenting with vomiting and abdominal distension, an obstruction can cause abdominal pain and, if involving the lower parts of the intestine, could potentially cause rectal bleeding.
Ectopic Pregnancy: In women of childbearing age, an ectopic pregnancy can cause abdominal pain and, if ruptured, significant internal bleeding, which might be accompanied by rectal bleeding due to the proximity of the reproductive organs to the rectum.

Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding, especially in younger patients. The pain might not always be localized to the LLQ.
Intussusception: A condition where a part of the intestine telescopes into another part, which can cause abdominal pain and rectal bleeding. It's more common in children but can occur in adults, often with a lead point such as a tumor.
Angiodysplasia: Vascular malformations of the gastrointestinal tract that can cause bleeding. They are more common in the elderly and can present with recurrent episodes of bleeding.