Your CYA of 20 isn't terribly low so I would just continue to use Trichlor pucks for a short while until you get to around 30 ppm CYA. During the winter, chlorine use is very, very low due to the cold water temperatures. One 3" 8-ounce puck of Trichlor will add 2 ppm FC to your pool and 1.2 ppm of CYA and I'll bet that you didn't add much more than 20 pucks over your winter, right? That would explain your relatively low CYA level -- of course, I don't know what CYA level you started with.

If you had winter rains, then your pool may have overflowed with rain water and that would have diluted everything in your pool, including CYA.

Also, several users have reported drops in CYA over the winter. If your chlorine went to zero, then there are anaerobic bacteria that actually consume CYA as food -- they are normally found in soil but can find their way into your pool. So that could have happened as well. Some users even report CYA drops over the winter even with FC that didn't go to zero, but that's still a mystery to me.

Richard