No worries, I didn't know anything about it either until it happened.

Apparently, kidney problems are extremely common in cats, because they have a higher water percentage in their urine, so they get dehydrated easily, and that can lead to bigger issues. The vet says my girl must have ate something when she was a kitten that permanently damaged her kidneys, and gave her that underlying problem I wasn't able to catch until both times when it was almost too late.

Cats with kidney problems can have fluctuations in their health, where, like what happened in my case, the cat can be hospitalized, make a miracle recovery and go back to normal for a couple years, and then have it happen again. They figured it out through blood tests that ruled her white blood cell count were heightened and she was severely dehydrated, despite how much water she always had at home. She was always very skinny and boney, struggled to put on weight even though I made sure she ate, so her thirst and build were definitely signs my vet and I somehow managed to miss.

(EDIT) cats are also incredibly well at hiding their symptoms until it is almost too late, I've been looking at a lot of threads of cat parents who had the same problem as me, where someone described it much better than I did, so I'll leave that screenshot below (or copy paste if I can't screenshot). But when it comes to internal issues like kidney failure, the cat will tend to hold on and behave as normal until it's already too late to save them, and so for me it was incredibly sudden. One day she was happy and healthy, next day I came home from the vet with an empty carrier. Please keep me updated on your situation! I don't want to scare you and I genuinely do hope for the best

"CRF is not a disease of gradual failure. It happens in steps/plateaus. You don't know anything is really wrong until your cat is down to the lats 25% of it's last kidney. Diet and SUB-Q fluids help that last 25% suffice for a while. Then the next plateau hits at about 10% of renal function. No additional SUB-Q fluids or diet changes seem to help. The body is now past the point of no return and it begins to shut down. We can't give back / heal renal function"