2001 Toyota Corolla Terrible fuel efficiency for a compact

I recently moved to the DC area, which is notorious for its gridlock. I have noticed a definite decrease in fuel economy from my normal 28 mpg for city driving to around 26. However, I just filled up my tank on Tuesday, and with school starting, construction projects in full force, and long traffic lights, I have noticed my gas gauge falling down to the first quarter tank mark at around 50 miles! Just last week, it had waited until I had driven 60 miles to fall to the 3/4 mark, but not anymore. I understand that rush hour traffic can have a profound effect on fuel economy, but I have had longer commutes in the past, with terrible traffic, yet still achieved 27 mpg (in Atlanta). I have fairly new O2 sensors, had my catalytic convertor replaced (my check engine light is still on, though, with a catalyst efficiency code PO420, probably because the convertor is an aftermarket convertor, and not a Toyota cat). I had platinum spark plugs put in to replace the old iridium spark plugs before I moved up to VA, but did not replace the fuel filter or PCV valve. I called garages, and they told me that I would not need either of those until I hit 150,000 miles. I keep my tires inflated, don't speed, turn off my AC when going up hills, and try to remember to shift to neutral when I am stopped at a long light. I also do not drive with my windows down. Nor do I have a roof rack, or haul heavy items, or do jack rabbit starts. Even more, I just purchased and used fuel injector cleaner from Walmart in hopes of it improving my mileage, and it hasn't worked at all. My question: why am I getting 20 mpg with such a tiny car? It's getting to the point where I am scared to drive now, because I hate getting such awful fuel economy. It is one thing to sit in an hour of traffic each morning and afternoon, but to go from 27 mpg to 20 mpg in such a short amount of time is a little harrowing. What is wrong with my car? Do I need a new mass airflow sensor? Seafoam? Fuel filter? Fuel induction system cleaning? I am trying to not spend much money, either.

Thank you for your help.