Sadly you unless you have a necropsy done you will stay in the dark when it come to the cause of death.
While it is unusual for an animal to just die it does happen, few years back I had an albino male (about a year, and had been in my care for 6 months) and I just found him dead one morning (healthy, shedding fine, eating).....cause of death aneryism.
So it can happen, does not mean you are at fault, there is also a chance the animal came in sick it's always a possibility as well.
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