I think that it was your cat’s time to go. It’s very hurtful when a beloved pet dies. I had a cat for 19 years and when he died, I was besides myself. I have other pets now, but I will always miss my dear Hector. Time heals all wounds.
Since it’s winter, is it possible he got into antifreeze? Reading up on it, it sounds like a very small amount (a teaspoon or two) will kill a cat.
He could have discovered/eaten a mouse that had recently ingested poison. It does sound like a poison situation…and it sucks. 