It might be possible. Any chance it was being harassed by another fish ?
However, even though you may not have an ammonia issue now, the fact remains that the number one cause of apparently inexplicable deaths is ammonia or nitrite toxicity. It does not have to be ammonia from today or yesterday.. it can be the result of exposure weeks or months before the fish actually dies.
If you had a spike at some point in the past, fish will be damaged in a very short space of time. Fish used in cycling, if you are not vigilant about the water changes, may be exposed to levels high enough to cause this type of damage. They may appear to be ok, you do a water change and all appears to be well and you think Ok, great, I caught it in time. But in fact the fish have been damaged, which results in much shorter life spans. Some number of days, weeks or even months later, the fish dies, and it appears to be for no reason.
It's also true that today, many fish are very inbred, which can cause genetic weaknesses we may not be able to see. Sometimes that can result in premature death as well.
It isn't the best idea to keep any schooling fish alone, for sure.
