Low coolant, probably just a very little bit low, probably the level getting really low as it contracts in the extreme cold and the heater core is the first thing to get no circulation and by the time it is warm and you check it then the level looks just fine. I would top off the radiator and the resivoir. If the radiator is low but not the res than you probably getting a little air in thru the cap, fill both res and rad and throw on a new cap see what happens. My 2011 2500 6.4 ram was doing the same thing a couple days ago... I also have an air door going bad but it just seems to be restricting the air volume not temp and makes loud clicking sounds when I start or shut off the truck, the air doors are all electric in these.