Why does it hurt to pass a small stone?

The ureters are actually quite thin, but they are pretty good at propelling urine down toward the bladder with peristalsis. Even a small stone can cause pain, which is from intermittent obstruction caused by the stone as it haltingly works its way down. There is a very good chance you will pass that stone without a surgery, but it may take a while. A small percentage of little stones (3 to 5 mm) get stuck in the ureter and require surgical treatment of some kind. The pain is caused by swelling of the kidney as the stone gets stuck and the urine backs up. Your urologist is probably expecting you to wait for a period of time and then followup – make sure you do that even if you pass the stone.