I'm so sorry to hear about your loss. I've had cats all my life and my motto has always been "cat acts out of the ordinary in any way" it's time to visit the vet. These creatures have a tendancy to go down hill so very fast when a problem arises so whenever I detect something, I always have to ease my mind by going to the vet. I'd still be really interested in knowing exactly what happened to your kitty though, but it definately sounds like UTI.
Again - so sorry about your loss, it hurts so much to lose a baby