The algaecide, being ammonia-based, will break down into several byproducts, ammonia included. The ammonia in itself can create a high chlorine demand, taking some of your useable chlorine away from its normal job of killing algae. That's why the only algaecide that we recommend here is polyquat 60, which is NOT ammonia based, and also works as a clarifier (same as your quick clear). Sounds to me like you had an algae bloom trying to happen. If so, the shock and algaecide that you added may have stopped it for now, but it won't for long.

If you could please post the test results that Watermom asked for above, it would help us help you to figure out what happened, and how to have it not happen again.