By completely cleaned out do you mean emptied 100%?
If you did not rinse the filter or gravel then chances are that there was still nitrifying bacteria alive amongst the filter media etc. This would mean the bacteria had a head start on the ammonia being produced by the fish and hence no ammonia readings now.
In future I would recommend you do just partial water changes on the tank. You will stress the fish less and have a lower chance of sending the tank into a mini cycle.