It has to be a coincidence. I can't imagine what the health benefits of a single 3-day fast could be that they would be so massive as to change your entire health for years.
A change in gut bacteria could potentially do that. However, normally that is quite difficult to change long term. I seem to have a collection of gut bacteria that give me bad gas quite often without sticking to a very strict diet and I found a few months ago that a five day fast reset that as well as antibiotics. However, either way it gradually returns to the way it was (most of the way back after three months). Good to know that three days of fasting helps some people, since that is much easier than five :/.
Quite possibly. However, if I found myself in a similar situation years from now, I would try it again.