Yes, narrow stools are quite common with IBS, although I'm not sure I have ever heard a theory as to why. Go to your appointment, but don't start sweating bullets over cancer. One, colon cancer is not very common, especially in younger people but IBS is quite common, especially in women. I think one of the ads here on the forum said as many as 1 in 5 Americans has some degree of IBS. Worrying over something worse than IBS will just make your diarrhea worse.