Live algae will clog a filter quick. I bet that's what you've got, just not enough for you to see the algae yet.

Test your water chemistry, preferably with a Taylor K-2006 or other FAS-DPD test kit. They can be purchased Post the test results here. If you have any algae starting to grow and chlorine in the water you will register combined chlorine on the test result. (Normally you would not expect to see any more than 0.5ppm CC) Here's a page on poolsolutions that tells you about he various test kits: http://www.poolsolutions.com/testkits.html

The solution will be to shock your pool until you register less than .5ppm CC and you also lose less than 1ppm FC between sundown and sunup (night because sunlight breaks down chlorine).