I'm so sorry to hear of your tragic loss.
The only way you might know the cause of death is a necropsy and after this time I'm doubtful your Vet will be able to do it. The ideas above are reasonable, even the cancer, even if you did not notice anything amiss. Cats, animals in general, are stoic and often do not show us signs of illness or pain till it's too late for us to act. I'm so sorry.