It sounds plausible to me. Try dosing back up to about 10ppm (shock level for zero CYA) tonight while it's dark, then test again in the morning before the sun comes up. That will allow you to determine if the chlorine is being consumed by sunlight vs fighting something. If you're around 9ppm then it was just the sunlight. If not, you need to get some CYA in there and keep your chlorine at shock level as best you can until you can complete that night time test without losing much chlorine.

http://www.poolsolutions.com/gd/best...e-chart.html#a

What are the rest of your test results?