Diverticulitis: The patient's symptoms of acute onset abdominal pain, particularly with radiation to the back and changes with position, along with rectal bleeding, are highly suggestive of diverticulitis. The improvement with positional changes and worsening with lying flat also support this diagnosis.

Internal Hemorrhoids: Given the rectal bleeding, especially with the first bowel movement and the use of Colace (a stool softener), internal hemorrhoids could be a plausible explanation. However, the abdominal pain and its radiation pattern are less typical for hemorrhoids alone.
Constipation with Fissure: The patient's use of Colace and the description of pain worsening during bowel movements could suggest a constipation issue complicated by an anal fissure, which could cause rectal bleeding. However, this doesn't fully account for the abdominal pain's characteristics.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis could present with abdominal pain and rectal bleeding. The acute onset and specific pain radiation might be less typical, but IBD should be considered, especially if the patient has other systemic symptoms or a family history.

Appendicitis: Although the pain's initial location and radiation are not classic for appendicitis, the severity and acute onset necessitate considering this diagnosis, especially given the potential for appendicitis to present atypically.
Intestinal Obstruction: The abdominal pain, bloating, and changes with position could suggest an obstruction, which is a medical emergency. The absence of nausea and vomiting does not rule out this diagnosis.
Mesenteric Ischemia: This condition can present with severe abdominal pain out of proportion to physical findings, and while less likely given the patient's age and lack of risk factors (e.g., atrial fibrillation), it's crucial not to miss due to its high mortality rate.
Ectopic Pregnancy (in a female patient, but mentioned for completeness): Obviously not applicable here, but always a consideration in females of childbearing age with abdominal pain and bleeding.

Intussusception: A condition where a part of the intestine telescopes into another, which can cause abdominal pain and rectal bleeding. It's more common in children but can occur in adults, often with a lead point like a tumor.
Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding and abdominal pain, usually in younger patients but can be considered in the differential for unexplained gastrointestinal bleeding.
Infectious Colitis: Certain infections can cause severe abdominal pain and rectal bleeding. While less likely given the acute onset and specific pain characteristics, it remains a consideration, especially if the patient has been traveling or has immune system compromise.