Backwash or clean your filter, might even have to replace the cartridge. It sounds like whatever junk the pool store sold you has clogged it.

Are you testing your pool yourself? What do you use? Generally it is recommended around this site to test yourself using the Taylor 2006 test kit, which doesn't contain a phosphate test because there is usually no need to test for that regularly. The kit allows you to test for free chlorine (FC), combined chlorine (CC), pH, total alkalinity (TA), stabilizer (CYA), and calcium hardness (CH). It is expensive but the reagents will last at least 1 or 2 years. I use it once a week and the rest of the time I test with a cheap HTH kit from Walmart.

We can help you more if we have the above noted readings, the size of your pool in gallons and what you normally put in your pool.