Doing any rolling? Or a particularly bad breakfall? (based on your search for tabi) Those are my alternatives to the kidney stone diagnosis.
On to the rocks. No, I've not had the pleasure. How is your hydration during your workouts? How is your hydration in general? I drink at least 32oz. at every workout, if not more, as well as a cup of water an hour before and half a cup fifteen minutes before. If you do have stones, no amount of water will dissolve them. That requires a change in the ph of your entire system, which can be done through diet. (That part I know because my cat had kidney stones.
) On a lighter note, my uncle thought he had kidney stones because he was passing 'bloody urine'. Turns out he was eating beets.
Let us know.