Flat Stool

I have had flat stools for the last 1-2 years. I've spoken with my doctor and have had 1 colonoscopy five years ago (came out fine, except for internal hemorrhoids), and a flex-sigmoidoscopy about a year ago (that came out fine too; tiny polyp which was removed). I just turned forty and was wondering what causes a "flat" stool, which I still have. Never ribbon-like or thin; normal color and width but definitely not round, more like it is flattened on one side. I have occasional pain on the right side too. Wondering if I should go back and do it all over again. Thanks for any info out there!