My fuel guage got below a half tank at about 3/8

1999 fuel pumps are located within the gas tank on this model and use a float assembly to send fuel levels to the dash gauge. This is why you may notice that parked on a hill or with sudden stops you car shows a little more or less fuel and then settles. It shouldnt go up and down as drastic as you describe so that makes me feel that the sending unit on the fuel pump is going bad. It looks like a half circle of contacts and as the float arm goes up each contact causes a different reading.. think clock face and clock hands. Watch it carefully now that you have filled it up and note when it gets to same range you were in before. If you see same problem it could be a failure.. if not then its possible a piece of debrs wash causing an intermittent failure. Unfortunately on these models the fuel pump and replacemetn is a lengthy process. You must drain the tank, remove it from vehicle and remove pump. insert new one and then do a pressure check at engine block as these new pumps have to maintain about 90PSI pressure to the fuel delivery system. Its not a weekend DIY fix to do this repair and if you see the issue its best to leave this job to a dealer or reputable service facility.