Hi again-
If the food you're talking about is K/D, that's actually for a different urinary tract issue than C/D is for. C/D is a great food for cats with UTI's, but K/D is targeted toward animals with renal insufficiency or failure... you may want to go back to C/D, if that's the case.
If you think it's a relapse, the easy thing to do is get a urine stick run to look for red and white blood cells. That's usually where I start out...
-Hobbit