That doesn't sound very good mate. I heard what Lottie had to say but in my experience of pets if it was a urinary (kidneys) failure, then its usually preceeded by the animal ******* itself everywhere often and its normally systematic ie it happens over time not as quick as you described.
Its more likely that your cat has had some sort of rear injury like falling off something or being hit by a car or worse some tosspot kid etc.
One of my cats got literally run over in front of my girlfriend, he managed to crawl away even though he'd been actually run over and didn't make a sound. It was the most awful and harrowing experience I have ever had with a pet, we had to get him put to sleep as it had smashed his pelvis and internal organs.
If I were you, i'd get your cat to the vet right now, the fact its not making any noise doesn't mean she's not in agony, i'm sure you wouldn't want that, if you have insurance it will cover it, get them to x-ray and get her sorted asap.
Mate i'm thinkin about ya as this is so horrible, I cried for a few days after my boy was gone. Get her looked at.