Why does my heater blow hot air then blows cold air. If I turn off the heater for 2 minutes then back on it blows hot air again.?

MY FRIENDS CAR DID THAT SAME THING. CHECK YOUR COOLANT LEVEL ITS PROBABLY EMPTY. YOU MAY HAVE A SLOW LEAK SOME WHERE. WHEN YOUR CAR IS RUNNING IT PULLS THE COOLANT THROUGH THE MOTOR IF YOU ARE LOW ON COOLANT, BUT NOT EMPTY YOUR AIR COULD TURN COLD. THEN WHEN YOU SHUT OFF YOUR CAR IT BUILDS BACK UP IN THE RADIATOR, SO WHEN YOU START IT AGIAN IT WILL HAVE ENOUGH FOR A WHILE UNTIL IT PULLS THROUGH THE ENGINE AGIAN. I HOPE THIS HELPS YOU.

the heater core has been flushed out. the antifreeze is full there is a new thermostat in it but it still does the same thing in the morning i have no heat at all. help please