I'm almost 20 and I've had recurrent problems with kidney stones since I was 15. If yours is only the size of a grain of sand, then you might have passed it already without even noticing. The biggest stone I've passed was 4mm, and I could feel it when it came out. You usually can feel it before you go to the bathroom too, because you have this sudden urge to pee and this pain in your vaginal area/urethra.
I'm not exactly sure why they would suggest that you should only eat soup, except for the fact that it's a liquid and might help along your stone. But there's nothing wrong with eating solids. Depending on what the composition of your stone is, they might tell you to refrain from eating things like chocolate and tomatoes, and apple juice. I have calcium oxalate stones, and I can't drink apple juice or cranberry juice, and there's a strange list of things I shouldn't eat a lot of.