Yes, you need to shock it. Since your Cl was so low before, odds are good that you have an algae bloom trying to happen (or there's something else in your water that is causing the large demand). I would shock the pool to 25 ppm, and hold it there until it quits dropping substantially overnight, then let it drift back down. if you do a partial drain/refill this weekend, then when you refill it, retest your CYA and adjust your shock level accordingly (see the best guess chart stickied at the top of the chlorine section).

Janet