If i had a colonoscopy 3 weeks ago, should i worry about seeing bright red blood in my stool?

Yes: Your endoscopist should be notified immediately and you should be examined.

Probably not: It is not uncommon to have some fresh bleeding after colonoscopy, especially if biopsies were obtained. Since the colonoscopy eliminates cancer as a cause, other causes are typically self limited. If bleeding increases or persists, check in with the md.