Embrace, before I offer any opinion, I want you to know I'm new to this hobby too, having started reseach on April 1st... So please DO NOT TAKE MY OPINION AS ANYTHING CLOSE TO "DEAD CERT."
My guess is that your tank bacteria was just at that borderline stage where they were just about coping with what should have been ~5ppm ammonia each day. Once you find that your ammonia and nitrite levels are reaching 0 within 12 hours, it is best to keep maintaining 5ppm of ammonia in the tank each 12 hours and ensuring it is processed to zero, likewise for nitrite, for the next 7days... KEEPING THE SAME UNCHANGED WATER. Apparently, this is known as a "qualifying week" and gives you good odds of your water being ready for your first fish after a big water change (~75-90%) the day befoe you goto the fish shop.
From what I've read, nitrite processing bacteria cultures grow much slower than ammonia processing bacteria at pH values above ~7.5, which possibly explains my own scenario in my QT (Southampton tap water is pH 8.2 and I'm having to do 50-75% water changes due to upto 0.3 nitrite readings, while my heavily planted Rio240 tank with a carbon dioxide bubbler is pH 7.5 and has never had nitrite higher than 0.1 since three days after introducing CO[sub]2[/sub]).
Another thing worth mentioning is that sometimes our water board companies will randomly change their treatment, that can mean that water from the tap comes with nitrite and nitrate spikes. It might be worth doing a test on your tap water.
One final thing that springs to mind right now is that have you accidentally added fresh untreated tap water to your tank that was looking so good? As in, have you forgotten to add your de-chlorinator eg. Nutrafin Aquaplus? The chlorine in tap water would wipe out you entire bacterial culture very quickly.
I had a similar incident to yours, but not quite as extreme with my Rio in late April... I initially was doing a "fishless cycle" by adding Sainsbury's "Tropical Flake" every other day. A week later I added plants and continued feeding an empty tank. A week after that I added my Dennerle CO[sub]2[/sub] system and my NH[sub]3 [/sub]and NO[sub]2 [/sub]values were looking good for ~7 days. Then out of the blue, I got a nitrite reading of 0.3, confirmed by my LFS and so I had to abandon my first fish for a few more days.
Hope you have not fallen asleep reading this driffle and its of some use, but please try and wait for more experience fish keeping input.
