I'd say a few days of 50 cya ppm increase is more than tabs too. In other words, Tabs + a powered shock w/ CYA in it, or accidentally adding Stablilizer. One more, the CYA tests aren't the easiest, but easy enough with the Taylor and TFT kits, but they must be done properly. Maybe just a testing mixup? On average, a 3" Trichlor puck will add about 3ppm CYA per week to your pool if dissolved. There is plenty of wiggle room there, but what I'm getting at is if you jumped it that high that quickly, there's a lot of tabs in there, or a powered "shock" product containing stabilizer, or both.