I am so sorry for your loss. I almost lost my cat two weeks ago, so I understand how hard that is. My Lizzie suddenly stopped eating and drinking, she had a high fever. The vet gave antibiotics and intravenous fluids. The test came back as indicating a liver or gallbladder issue. At first she still didn't eat, I had to force food and water. She started getting better a few days after the vet visit and an ultrasound showed no issues. I realized she was on the mend when she started milking the attention. They said she had a gallbladder attack, likely something she ate that didn't agree with her? I have switched to a higher quality food, apparently some cats cannot tolerate the traditional commercial cat food. The vet said it's like McDonald's food, it tastes great, but there are better foods out there, regular commercial cat food is high in acid. I also used to give her a lot of treats, she loves her treats, and would do tricks for them, but they are higher in fat. So, Trader Joe's cat food, holistic treats and not many of them.
It's hard to say what went wrong with your cat. Possibly she ingested a poison or toxin somehow? Or she is one that cannot tolerate cheap cat food? It could be she had a genetic defects. Who knows?
This whole thing made me aware of how attached I am to my cat. I have always been an animal lover, but my husband works long hours and she is my companion. She is smart and funny and loves me madly. She works hard at communicating her wishes, plays little jokes on me (grabbing my ankles under the bed, sneaking up and giving me a scare, etc.), she even enjoys music. I almost want to distance myself from her so it won't be so painful if something happens, but I don't think I have it in me.
Pets really worm their way into your heart that's for sure, so I get how hard this can be.