Can Kidney failure come on fast?

I know this is too late to ask as Kira died on Thursday.

But I am still stunned and trying to figure out how we missed it.

She was diagnosed with Immune mediated anemia, and treated successfully
for it. Back in August, her bloodwork was back to normal, her weight
back up, and her activity back to normal. She looked great, and we
thought everything was fine.

She started losing weight. She was still eating, just not as much. Last
Sunday, she stopped eating. The first day, I thought she was tired of
the food, so I was trying to switch it up. I had a back injury, so I
went to the doctor on Monday. She was still not eating that night, and
seemed to be hanging out by the water bowl, so we went to the vet
Tuesday morning.

She had some goop (not there before), so the vet figured she had a
respiratory infection again, and her gums were really pale. So he took
a blood sample and prescribed the prednisolone and the antibiotic
again, which worked great before. She was also dehydrated, so they did
sub-Q injection there and I took a pack home to continue. I also got
some canned food, which I mixed with water to give her with an eye
dropper.

The next day, the vet told me the bloodowrk was a little worse than
last time, and confirmed the prescription dosages.

No improvement on Tuesday or Wednesday. On thursday, as we were heading
to the door to take her back to the vet, she died.

When I was doing my online research during the last round, I really
couldn't find any big serious issues that cause immune mediated anemia.
Looking at different symptoms this time, it sounds like she had kidney
failure which would explain the lack of appetite, dehydration, and the
excess thirst. But that implied non-regenerative anemia, which was not
the case with Kira's bloodwork. She had a geriatric panel in
February, a regular full panel in July, and CBC panels 8/4, 8/11, 8/29,
and 10/17. All of them said regenerative. And her full panel in July
had normal levels for BUN and creatine (and most everything else).
Could it go bad that fast?

I know it's too late. I just can't understand what happened. She seemed
so healthy. A week ago, I was just beginning to get a feeling that
something was wrong. She was active and happy. Good weight and healthy
coat. The first time, she was just barely anemic when I figured there
was a problem and took her in. This time, she went from looking great,
to being dead in a week.

I know I missed the excess urine. They all pee in the same corner or
each box, so I just assumed it was multiple cats. I do the downstairs
boxes, and my mom does upstairs. I didn't think to ask if the upstairs
box was getting less business.

I won't have a senior again for several years, but what should I be
looking for? My first cats lived to be 18 and 19, so losing Kira at age
12 just came out of nowhere. I thought I was being proactive by doing
annual bloodwork and by taking her in again in July and requesting new
bloodwork. But it didn't catch anything aside from the anemia which
doesn't seem to be the only problem.