Maybe the bacteria in the filter got wiped out. Did you put anything anti-bacterial in the water or clean the filter with it?
The only thing I've changed is my RO supplier, I don't have my own unit.
RO water comes up at 2ppm, but 0 ammonia.
I tested the water test on monday before I did a water change. I thought the ammonia test came back as 0, but maybe I was wrong. So i'm not sure if the problem was there before or after the water change.