One other comment: there are weird things that will cause actual diarrhea, such as fructose intolerance if you're suddenly eating fruit for example, and/or various bowel disorders But, generally, it's just your intestines going, "Oh crap! (no pun intended) What is all this?"
It would be no little different than if you suddenly started taking a bunch of Metamusil.
Remember: you do want a couple of bowel movements per day.
Also, fiber has done VERY well in the studies. I hope to do a page on that sometime soon. It's downright miraculous. (I mean fiber in a natural, whole foods diet of course.)
One last thing: I have heard of people having a "putrid bowel" depending on thier dietary history. It may take some time to heal if this describes you...