Irritable Bowel Syndrome (IBS): Given the absence of elevated infection and inflammation markers, and normal calprotectin levels, IBS is a plausible diagnosis. The presence of abdominal pain and occasional blood in stools, which could be due to hemorrhoids or anal fissures often associated with IBS, supports this consideration.

Inflammatory Bowel Disease (IBD): Although the normal calprotectin and lack of elevated inflammation markers make this less likely, early or mild cases might not always present with significantly abnormal lab results. The occasional blood in stools is a red flag that warrants consideration of IBD.
Diverticulitis: This condition can cause abdominal pain and bleeding, especially if there's a diverticulum in the sigmoid colon that's inflamed or has eroded into a blood vessel. However, it's less common in younger individuals without other risk factors.
Anal Fissure or Hemorrhoids: These conditions can cause pain and bleeding, especially with bowel movements. They are common and could explain the symptoms, especially if the bleeding is minor and the pain is localized to the anal region.

Colorectal Cancer: Although rare in young individuals, it's crucial not to miss this diagnosis. Any rectal bleeding, even if occasional, warrants a thorough investigation, including possibly a colonoscopy, especially if there's a family history.
Infectious Colitis: Certain infections can cause similar symptoms and might not always present with significantly elevated markers of infection or inflammation initially.
Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause bowel obstruction and ischemia. It's rare in adults but can present with abdominal pain and bloody stools.

Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding due to the presence of ectopic gastric mucosa. It's rare and usually presents in childhood but can be considered in young adults with unexplained gastrointestinal bleeding.
Vasculitis: Conditions like Henoch-Schönlein purpura can cause abdominal pain and gastrointestinal bleeding, though they are rare and typically present with other systemic symptoms.
Gastrointestinal Lymphoma: A rare malignancy that can cause a variety of gastrointestinal symptoms, including abdominal pain and bleeding.