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Since you have had the abdominal pain & blood in the stool for a long time, you should have colonoscopy. It could be Crohn's disease although usually it causes diarrhea. Diverticulitis is possible but it is rare in your age.
If you have Crohn's disease, you would ahem the complications such as bowel obstruction, fistulas between the intestine and another organ which could be life threatening.