You should go to a radiator shop and have them pressure test your system, or drop the $100 and get your own tester. Same thing happened to me a few weeks ago, went to check the coolant and the radiator was half emtpy and no sign of leaks. Went for a pressure test and my heater hoses started spewing. Drove over to Napa and now all is well.
So it's not the head gasket no a drop of water in the oil after a change yesterday