Diverticulitis or Diverticulosis: The patient's symptoms of rectal bleeding, abdominal pain, and changes in bowel habits (initial diarrhea followed by constipation) are consistent with diverticular disease. The presence of bright red blood in the stool and on toilet paper further supports this diagnosis, as diverticulitis can cause bleeding from the diverticula in the colon.

Colorectal Cancer: Given the patient's age and symptoms of rectal bleeding, changes in bowel habits, and abdominal pain, colorectal cancer is a plausible diagnosis. The presence of bright red blood could indicate a tumor in the distal colon or rectum.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis could explain the patient's symptoms of abdominal pain, changes in bowel habits, and rectal bleeding. The initial presence of diarrhea with phlegm and subsequent constipation could indicate an inflammatory process affecting the bowel.
Angiodysplasia: This condition involves vascular malformations in the colon that can cause rectal bleeding, especially in older adults. The bright red blood in the stool could be consistent with angiodysplasia, although it is less likely than diverticular disease or colorectal cancer.

Ischemic Colitis: Although less common, ischemic colitis can present with sudden onset of abdominal pain and rectal bleeding. It is crucial to consider this diagnosis due to its potential for severe complications if not promptly treated.
Infectious Colitis: Certain infections can cause severe colitis, leading to abdominal pain, changes in bowel habits, and rectal bleeding. Identifying and treating the underlying infection is critical to prevent serious outcomes.
Gastrointestinal Lymphoma or Other Malignancies: While less likely, other malignancies affecting the gastrointestinal tract could present with similar symptoms. Missing these diagnoses could have significant implications for patient outcomes.

Intussusception: A condition where a part of the intestine telescopes into another, which can cause abdominal pain, changes in bowel habits, and rectal bleeding. It is rare in adults but can occur, especially in the context of a lead point like a tumor.
Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding, especially if it contains gastric mucosa. Although more common in children, it can present in adults, especially if complications arise.
Solitary Rectal Ulcer Syndrome: A condition characterized by ulcers in the rectum, which can cause rectal bleeding, pain, and changes in bowel habits. It is less common and might be considered if other diagnoses are ruled out.