Yes, the "sudden drop" you describe comes from the biodegradation of CYA, of which the main waste product is ammonia, which as we all know, is the main waste product in urine. So, you basically have a whole buch of wee beasties peeing in your pool. Eventually, you will burn up all that waste product, but it will take awhile. The real test is holding Cl overnight. That way you eliminate the effects of the sun on your Cl levels.

Michael