Sorry about your loss hon. Sadly in this hobby we often don't know the cause of death when we lose fish. They have can have the same internal issues as any living creature, she could have had a heart attack or liver failure or something, and we'd never know without a fish necropsy, and that's rarely worth doing unless dealing with very expensive or rare fish.
So sadly, we usually have to accept that we'll never know the cause of death.
But we can say that it's highly, highly unlikely that you did anything, or didn't do anything, that caused it. You did the right things, by doing the large water change before you left, 3-4 days without food for fish is nothing to worry about at all, so being away didn't cause starvation or anything, I promise - and without food going into the tank from a defective auto feeder or something, an ammonia spike is unlikely, and I don't see redness on the body that is often seen with ammonia and nitrite poisoning either.
Do another large water change simply because there's been a death and a decomposing body in the tank for a day or so, and test some of that old water once you're done, but even if there are traces of ammonia or nitrites when you test, that may be as a result of the death, and not the cause. But I'm predicting your water tests will come back fine.
Sorry hon. It sucks, but it happens, and we've all been there and empathise with you!
