What Causes Chronic Diarrhea?

I have been having a problem with my bowels. Some days I have real diarrhea all day long.Some days like today no matter what I eat I start going all day long so far today I have been up since 7 am and I have already gone aat least 6 or 7 times. Now I am begining to have some blood which I think is from hemrroids. The blood is minimal and will stop as soon as I stop. I did see a specialist and he took samples and said that there were no parisites that he could tell. That was all he said and made no other appointments