Another Coolant Problem

[UOgarath]

I have a 1999 2.2L Cavalier RS. Last week my low coolant light came on and went back off. I checked the coolant level and it was very low. I had a gallon of dex-cool antifreze in my trunk so I pulled over and filled up the overflow resivoir to the arrow after the engine cooled down for about half an hour. Today the light came on and stayed on. I found this odd so I checked the coolant level again and the resivoir was nearly empty again. I had bought another gallon of coolant pre-mix so I waited for the engine to cool and filled to the line again. I made it home, I havn't checked the level yet waitng for it to cool. I checked everywhere that I have stoped there have been no puddles under the car. The air and heat work fine so I know it isn't the heater coil. My temp guage is at it's usual place (just under 195). I'm not ruling out the thermastat though but does anyone know what is wrong before I try replacing the thermastat?