Hemorrhoids or Anal Fissure: Given the age and presentation, a common and benign cause of rectal bleeding in adolescents is hemorrhoids or anal fissures, often due to constipation or straining during bowel movements.

Gastrointestinal Infection: Infections such as gastroenteritis can cause rectal bleeding, especially if the infection is severe or caused by a specific pathogen.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with rectal bleeding, although they are less common in this age group without other symptoms.
Rectal Prolapse: Though less common, rectal prolapse can cause bleeding, especially if there's a history of constipation or straining.

Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause bowel obstruction and ischemia. It's rare but requires immediate medical attention.
Infectious Colitis: Certain infections (e.g., Clostridioides difficile, Salmonella, Shigella) can cause severe colitis with bleeding, which needs prompt treatment.
Trauma: Sexual assault or accidental trauma can cause rectal bleeding and must be considered, especially with a history of "something in the past."

Vascular Malformations: Rare conditions like arteriovenous malformations can cause rectal bleeding, though they are uncommon in adolescents without other symptoms.
Tumors: Though exceedingly rare in adolescents, rectal tumors (benign or malignant) can cause bleeding and should be considered if other causes are ruled out and bleeding persists.
Coagulopathy: Bleeding disorders can manifest with rectal bleeding, especially if there's a family history or other signs of bleeding tendency.