With a negative colonoscopy, it is unlikely that a polyp or cancer is causing the bleeding. Hemorrhoids or fissures can be missed with colonoscopy - you may want to consider an anoscopy for further visualization. It is possible that the colonoscopy may have missed something more serious, but it is unlikely.
IBS may be possible - although bleeding is not necessarily common. Certainly the aspirin may cause any hemorrhoid or fissure to bleed. You have to discuss with your physician the risk/benefit ratio of taking aspirin versus stopping it (i.e. the cardioprotective effects vs risk of GI bleed).
If the bleeding is a concern, you may want to consider switching to Plavix, which may have a lesser bleeding risk.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Thanks,
Kevin, M.D.