When you changed it all out you abviously lost some coolant. When you filled it up and topped it off did you let it warm up and make sure it started circulating before putting the top back on? Usually what happens is once the thermostat opens up the coolant level goes down because it fills up the air that was gain when the change was made. pop you rcap back off and make sure it is till topped off. Good luck, if it's not that then like they said, the thermostat is most likely in backwards, don't worry it's a common mistake!
