Lactose Intolerance or Milk Protein Allergy: This is the most likely cause because lactose intolerance can lead to inflammation and irritation in the digestive tract, potentially causing microscopic bleeding. Similarly, a milk protein allergy can cause an immune reaction leading to inflammation and bleeding in the gastrointestinal tract.

Gastroesophageal Reflux Disease (GERD): Although not directly caused by dairy, GERD can be exacerbated by dairy consumption in some individuals, leading to esophageal irritation and potential bleeding.
Hemorrhoids: Dairy products can be low in fiber, and a diet lacking sufficient fiber can lead to constipation, straining during bowel movements, and the development of hemorrhoids, which can cause blood in the stool.
Anal Fissures: Similar to hemorrhoids, anal fissures can be caused by constipation and straining during bowel movements, potentially leading to bleeding.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause bleeding in the stool and may be triggered or worsened by certain foods, including dairy products. Missing this diagnosis could lead to significant morbidity.
Colon Cancer: Although less common in younger individuals, colon cancer can cause blood in the stool. It's crucial not to miss this diagnosis, as early detection significantly improves prognosis.
Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood supply and causing bleeding. It's more common in children but can occur in adults, especially if there's a lead point like a tumor.

Milk Protein-Induced Proctocolitis: A condition primarily seen in infants, where they develop blood in their stool due to an immune reaction to cow's milk protein.
Eosinophilic Gastroenteritis: A rare condition characterized by eosinophilic infiltration of the gastrointestinal tract, which can be triggered by food allergies, including dairy, and may cause bleeding.
Angiodysplasia: Vascular malformations in the gastrointestinal tract that can cause bleeding. While not directly related to dairy consumption, it's a rare cause of gastrointestinal bleeding that should be considered in the differential diagnosis.