I suspect that some of your CYA was consumed by soil bacteria that got into the pool, especially if the pool was "let go" over the winter. Such bacteria convert CYA into ammonia and it takes a lot of chlorine to get rid of ammonia and initially it will increase CC levels with FC registering zero (which is what you saw). In the future, you can buy an inexpensive ammonia test from a pet/fish/aquarium store and figure that you'll need to add a cumulative FC that is 10x the amount of ammonia (measured as ppm Nitrogen).

Richard