Trucked in water didn't have CYA in it -- it's NEVER used in potable water treatment.

Stabilizer dissolves slowly, so what you added 4 days ago is only now fully dissolving. If you added granular stabilizer yesterday, you'll probably continue to see a rise in CYA levels over the next few days.

As a rule, a drop in filter pressure means a drop in flow, often because of a dirty strainer basket, or a clogged impeller. Is that a possibility on your pool?