Could be parasites. Can go in for a test yet is may not show up. It wouldn't hurt to do a colon cleanse program. There are several on the market, I personally have had great results with drnatura's colonix. You could also consult with a naturopath if you are not getting answers from your doctor. Good luck. Let us know what it ended up being.
light-colored, kidney bean shaped pieces in bowel movement?
Think I finally have the answer (at least for me) to this. I take various pills each morning including several that look like a kidney bean. I read on another site that it could be a large enteric-coated pill that somehow makes it through both intestines while various "new coatings" adhere to it. This makes perfect sense to me as it never occurred before I started taking these pills.