Well you were lucky there was nothing wrong for that long. Water evaporates, but chemicals in the water do not, they just keep building up.
It's like if you wouldn't change oil in your car. It will still run for a while until one day......
I would suggest to take a sample to your local LPS and have it tested for everything. water doesn't change the colour if there is high level of ammonia. That's why there are aquarium water tests. If you have perfect bio system set up with right amount of live plants you would be probably ok.
Oh I almost forgot .... you need to take out carbon from your filter if you are using any medication.