Hi again,
There is a long mechanism for stone formation, much of it involving an alkaline urine, (slightly acid keeps the minerals in solution) which is usually related to the nature of the food, as it is usually the controlling factor to urine pH. It isn't the 'ash' content, as thought before, more the balance of proteins and carbs. But this still isn't answer. I don't think I know the whole answer any more because it is relatively immaterial. You get stones, you take care of it. Prevention is not a rewarding enterprise.
Do you know this was a clot for sure, or maybe just a concentration of blood cells that you assumed was a clot. Left to stand, blood settles out and if there is a depression, may seek the lowest spot. But you were there, not I.
Stones can go on for a long time without problem. Unless this happens again, or you get more overt signs, don't worry.
We often think spaying is the end of reproductive tract problems, but that depends how the sterilization was done. If your vet did the surgery, ask if the whole tract was removed, or just the ovaries. I suspect it was uterus and all, but it doesn't hurt to ask. This doesn't mean she will come into heat again, just that, if so, there is another potential source of blood, but no evidence that this may be the source...again, yet.
I can appreciate your anxiety, but until we get more symptoms to go on, trying to determine the cause can be expensive, time consuming, distressful for the cat, and, in the end, possibly unrewarding.
I'm about out of ideas to go on. Let's wait till the next time, if there is one, and go from there?
Please check just one ACCEPT box. Feedback is nice, too. Medicine comes in doses of Science, Art, and Miracle, given as needed.