Yesterday my 97' broke down and had to be towed. At the shop they said that the fuel sender had gone bad causing the fuel pump to burn up. The job was near $1000, thank god for the warrenty. Anyways, they only filled the tank with two gallons of gas. Not to mention the car no longer performes the way it did. So, I filled the tank up and the performance improved slightly. The one thing that worries me is now whenever I loosen the gas cap it hisses like a soda bottle being opened. Is my gas tank under pressure and what could be causing this?

your gas tank isn't under pressure, it'* under a vacuum. Fuel is being drawn from the tank but not being replaced by air, thereby creating a vacuum. That also puts a load on the pump. Are you sure they put the correct (vented) cap back on? If so, look at the purge valve/solenoid. It'* part of the evap system in the manual (if you have one).