If the K&N air filter is the reusable one, I believe that IS your problem. The oil off the filter media, fouls the MAF sensor which results in 2-3 mpg loss. It happens pretty suddenly and stops pretty suddenly when you clean the MAF and put a paper filter back in.

If the MAF is failing (voltage out of range) you usually get P0170 and P0173 OBD codes. But if it's fouled no codes just lower gas mileage.