This has pretty well been covered with the above responses. But I have had IBS for over 15 years and have had many instances of just mucous and more times than not it is accompanied with some blood. I can usually tell when its going to happen these days due to the odd "crampy" icky feeling I get in my gut. And usually I feel pretty ill for a few moments afterwards.
I think that IBS is just a catch all diagnosis for when doctors can not find anything wrong with us. Not sure about anyone else, but my symptoms seem to morph and change constantly and the triggers seem to be change as well. I have gone through bouts so bad that I have actually written letters to my children to open when they get older saying daddy loves them because I felt sooooooo bad and the doc could not find anything wrong.
Sorry for the off subject rant, just fed up with my health these days.