This symptom always causes alarm. However most occurrences will have a benign cause.
Polyps and cancers can both cause rectal bleeding, often small amounts of blood, and this can be associated with a change in bowel habit.
Haemorrhoids are large rectal veins which can bleed. This bleeding is often large but not serious.
Diverticular disease is very prevalent as age increases and every diverticulum has a blood vessel that could bleed.
Often colonoscopy is advised for this symptom.