Why does my PUR water filter blink red after a while, even though I haven't used it at all?

It's the type of filter that attaches directly to the faucet. There's an option to route the water through the filter, or let it go directly through the faucet. After I changed the filter once, I have only ONCE turned the knob to let it go through the filter (for drinking water). However, after a while, when I decided to route the water through the filter again, it blinked red, telling me to change it. How can this be if I used it only once? It's not like the filter uses biological components that expire.Why does my PUR water filter blink red after a while, even though I haven't used it at all?Room temp is the problem,.