Cat sudden death

Hi. My two year old cat Melanie was fine and two days ago she became lethargic and her temperature was 35. We took her to the vet, she got blood work done, fiv and felv tested negative. The x-ray showed pneumonia so they gave her antibiotics and tried to warm her up. Her temperature dropped to 33C. An ultra sound revealed fluid in the lungs, not a lot. They sedated her and when taking the second seringe of fluid her heart stopped. Her blood work was perfect, she had no symptoms no cough, discharge nothing. She wasnt exposed to anything toxic and the autopsy showed no cancer, no enlargment of the heart. It wasnt PIF, they tested with vinegar and fluid was transudate. I can’t understand and the vet can’t either. What happened? what caused this?

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