my mpg decreased?

i took my focus in for scheduled maintenance (oil change, tire rotation, and systems check) last month and immediately afterwards (as in on the way home) my mpg decreased. the ongoing mpg is shown on the dashboard but i also check after each tank refill to ascertain accuracy. for an entire year since i bought the car new last december i have gotten 32-34 mpg town/42 highway on every tank of gas and after servicing i started getting 29-30 mpg town (haven't taken it out of town yet so don't know about highway mileage). i am driving the same way i always have but, now, no matter how i drive i can't get above 30 mpg. i asked the service tech what might have been done that could cause this change but, of course, was treated like i grew another head and was told that nothing they did could have caused this...but, the fact is that the service was the only thing that changed. can you think of anything that could have caused this? whatever happened, i want my old mileage back. thanks