OK. First. I know it's not easy but don't panic.
The chances are she's got an infection which the vet will sort out.
If, on the worst scenario, she has kidney problems then there's a lot vets can do for that nowadays.
When Lucy was at the vets this week I was speaking to an elderly couple who had a 16 year old Birman cat (and beautiful she was too
). She'd had kidney failure for 3 years and was still going strong. She was at the vets for her check-up and repeat meds.
There's so much can be done nowadays for cats with kidney problems - and humans too. My 19 year old son has been on dialysis since his kidneys stopped working when he was 19. He hasn't had a transplant yet (hopefully next year) and is absolutely fine.
The Birman with the kidney failure really didn't look like a cat with any problems at all, so take heart and try not to worry too much (and I know that is the hardest thing in the world).
Will send out healing vibes for you and your little cat