Ginger,
There are urinary track infections that do not show in the first level tests. Go back to your vet and ask him to run the second tier of tests, explain the problem. Our Maude is prone to UTI's -- nothing is funnier then watching me walking her waiting for her to squat, then lifting her tail and shoving a pan under her.
You might try giving her a cranberry pill in the morning. That has worked not only for me but for both Maude and Casey.
Tea