If you use a bicycle machine at the gym or just a regular one, that can definitely aggrevate hemmroids and keep them from healing up; it's the way you sit on the seat. Also, chronic constipation can keep them stirred up, as can a lot of diarrhea that keeps you on the toliet a long time (the way you sit on a toilet puts pressure on your hemmroids--the more you sit, the worse it is).
If you have had a colonoscopy since you started bleeding, then you're probably okay trusting your doctor's opinion that you just have IBS. If the bleeding is more recent, though, then you need to go back to the doctor and tell him about it.
If he gave you pictures of your colonoscopy, you may be able to see if you have hemmroids; one of my pics is clearly labeled "internal hemmroids". That's how I knew for sure that I had them, although I suspected them when I would occasionally bleed a bit after a bad constipation spell.