Disappearing gas...

I just got my cars back from the shop from having it's power steering rack replaced. I noticed that it was low on gas and went to fill it up. I used 93 and filled it to about 3/4 of the tank. I drove around town to run a few more errands and I noticed that the gas seemed to the burning a lot quicker then usual but it didn't seem like a big deal. I parked the car over night and today as i was driving it it started to do some really weird stuff. the gas gauge and the gas computer just out of the blue dropped from a half tank to quarter tank. The computer display went from 150-160 miles left to 40 miles left with-in about 5-10 mins of driving. I was not driving fast or doing anything out of the ordinary just regular city driving. The car doesn't seem to be leaking all thought if it was i wouldn't really know where to look. I checked it out and i dont really smell any gas leaks either, once again i dont know if it was leaking if i could smell it either. Anyways does anyone have a clue as to where the gas is going? I was told that my air filter might need to be changed but that seems like a lot of gas to be burned in such a short amount of time for a bad air filter. Plus is just stared happening too. Any help is appreciate.