Also,

organic matter + no Cl + sudden low CYA = HUGE CL DEMAND

I've had this happen to me, and Ben wrote a post on what happens. Basically, with low Cl, there is enough of the bactieria present that can break down CYA. The big problem comes from the waste products of these bacteria...they create a enormous Cl demand. It will take lots to clean up the mess, but it will eventually slack off. Definitely get whatever gook you can out of there. That will help for sure.

Michael