Something seems to be consuming it. Not sure what it is but I'd try to keep it up around 15 for a couple of days more, maybe you'll win. The PH problem baffles me as well, especially since your using only bleach. Maybe we'll get lucky and the "Doc" will take a look.
As far as using the pucks to get your CYA up, that's an excellent idea except they will also lower your PH. You'd better keep a close eye on it if you go that route and keep plenty of borax on hand.