Well, if they're able to move, and a 2 mm stone is small enough to be mobile if it formed in the bigger tubes of the kidney. They normally do pass fairly quickly. The urinary system is designed to filter out and pass things through quickly. If you're watching carefully, you can sometimes see when the stone comes out, and normally you feel the irritation as the stone progresses because it irritates the bladder and urethra as it moves through those areas. It wouldn't be unusual at all to have pain in your lower back left over after you had the stone, but I guess if it got lodged in a tight tubule, it could still be stuck in your kidney. The longest-lasting one I ever had lasted about seven hours max from onset of initial pain to the stone's exit.