Litter box odor

My cat is a spayed two year old female with no health problems. She eats heartily (Royal Canin and Eukanuba dry) and drinks filtered water. She is also regular in her no toilet habits-no problems with defecation or urination.
That being said we notice that her urine seems to have "sour" or "sickly-sweet" smell. Cold this be a chemical reaction to the litter we have started using (World's Best Cat Litter) or something else? I will be taking her to the vet for a regular examination next week and I will mention it to her then. In the meantime, does anyone out there have any ideas?

I am sorry to say but this sounds like the first symptoms of Diabetes, please don't delay,please get your cat seen immediately- I am moving this to our health forum. oops someone else must be moving it as I type-

Fortunately it did turn out to be a chemical reaction between not two but three different things. My cat's urine was interacting with the litter-it had never done this with any other. The third offender was a plug-in deodorizer that potentiated the effect of the former reaction. I changed the cat litter, removed the plug-in and voila-no more odour!