A tumor (or something else) obstructing the hole in the middle of the rectum/lower colon would cause thin stools 100% of the time. You wouldn't have normal times and thin times. Just thin stools EVERY stool.A tumor doesn't come and go obstructing the colon sometimes and not others. Either it is, or it is not.Bascially in IBS any imaginable (and heck some would say unimaginable) stool alteration is seen. Stool size, shape, color can vary substantially in normal people and with IBS that tends to happen more, and when people are in pain with a GI problem they tend to look at the stool more as well.In a nutshell IBS is altered stool consistancy or frequency that is associated with pain and discomfort when other disease states are not present.So it could be IBS. For most age groups IBS is much more common than colon cancer.K.