Well as convict knows I don't need to change my water and I am not sure why. Well the longest I've gone is 6 weeks but normally I change at 4 weeks just to clean it up. Water always test fine, just this week I decided I'm changing 10 % every week for so on so I'm reasons. If you want to know check out my thread water changes.
Could this be that you did to much of a change and cleaned a little to well and messed up the natural balance. And then waited to long to re change water. What I am saying is you cleaned to much good bacteria and the tank could not do its job without your help. Make since?