What is wrong with my car.please help!?

i have a 99 monte carlo. it was fine saturday night and then wen i drove it sunday morning it was spittin and sputtering. wen i got out the store it wouldnt start. now it starts and the idle immediatly drops in 2 to 3 secs and the car shuts off. if i try to give it gas it immediatly shuts off. i changed the fuel filter already and it still does it so i think its the fuel pump.but idk. please help! i have had no problems wit the car before it starts up perfectly and runs real good.