I've had bleeding in stool once in awhile. i've had an upper gi and normal abdominal/pelvic ct. does that detect cancer? should i still do a lower gi?

Probably: In general, bleeding from the GI tract needs evaluation. Some common things like hemorrhoids or tears can do it. At your age, colon cancer is rare, but not impossible. Other problems like inflammatory bowel disease (crohn's) or even celiac disease can do that. If the other studies are negative and you are still having bleeding, a colonoscopy is very reasonable as a next step.