I'm so sorry for your loss. Something very similar happened to me a few months ago so please don't be too hard on yourself wondering what could have happened. I agree with having the necropsy done just to know what happened though if it eases your mind. You said she was just over 2. So was my cat. In fact, we had had him since the day he was born so he was exactly 2 years, 1 month and 2 days old. I have 11 cats right now and had 10 at the time. Out of the 10 he seemed to be the healthiest and most energetic one. About 2 weeks before he died he seemed a little off, but the ER vet and the regular vet couldn't put a finger on what was wrong. They discovered that he had polycystic kidney disease, though that likely wasn't what was causing him to feel bad at that point. A week after my first vet appointment, I brought him back in for some more tests. And then right there at the vet, he suddenly collapsed and died. I'm thankful that it happened at the vet and not at home, but it was still such a shock - to me and the vet. That morning he had been acting completely normal. He was running, playing, eating - acting COMPLETELY normal. And then 2 hours later he was dead. The vet attributed it to undiagnosed HCM - heart disease. Apparently heart disease is the #1 cause of sudden death in cats. I was miserable and I felt guilty, but he honestly hadn't shown one single sign of heart disease until he died. Even 2 weeks prior when he was acting a little off, he just seemed a little lethargic, but nothing that would have specifically diagnosed heart disease.
I did a lot of reading about HCM and I found one website (that I can't seem to find again right now) that gave the average age that a cat with HCM will live. This isn't the exact number, but if I can find the page again I'll post it, but it was something close to just over 2 years old. Granted, thats an average and not true for every single cat, but my cat was about 2 weeks older than the average age to die from HCM.
So while I obviously can't diagnose your cat, just know that you're not alone, and you're not the first person this has happened to. Its scary to me because now I've become overly paranoid about all of my other cats because I just have no idea if any of them are sick. Even with regular vet appointments, I still didn't know that my cat was sick.
It may not have been heart disease, but its very possible. And sadly, both animals and pets can just collapse and die very suddenly. Back in 1994 (I think it was 94, or somewhere close to there) there was a Russian figure skater who suddenly collapsed and died on the ice one day. He was only in his 20's. Up until that point he had been healthy and active and completely fine, but when he was autopsied it was discovered that his heart was so bad, that there was only a pin prick sized hole for blood to flow through, and ultimately, thats what killed him. I'm not sure why that story is sticking in my head right now, but I guess my point is that sadly, animals and people do often go undiagnosed with diseases, and its no one's fault because the signs and symptoms just aren't there. And cats especially are very good at hiding when they're sick.
So once again, I am so so sorry for your loss. Your little girl is probably playing happily with my Meepers at the Rainbow Bridge now

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