For the last couple of weeks, I've been getting an indication of combined chlorine when I do my drop test. The free chlorine was at 4.5 today. It only takes 1 drop of the FAS/DPD reagent to return the sample to clear, so that's telling me that there is .5 combined chlorine. Is this a problem? I've never had it before. I'm wondering if I should shock to pool to eliminate it?