Here's an odd one, any ideas?

-been super cold in NW Iowa lately, so I've been warming up my Ram (2011 Ram 1500 Quad Cab 4x4, 5.7L Hemi V8) for 10-15 minutes in the morning before taking off.
last week (Thursday), I had to make a couple of stops before work. First stop was about 3 miles away, so I started her up, ran for 15 minutes and got in, no heat (still blowing cold air when turning on the fan). Drove the 3 miles to first stop, shut off and went inside. Inside for about 10 minutes, then came out, started Ram again, and nearly instant heat (drove onto to work about 10 miles away, plenty of heat, working normally). Didn't think much of it (pretty cold day, -15 or so that morning) and went on with my day. The next day, up early, straight to work this time. 15 minute warm up, and got it, no hear. No heat at all going to work (temp gauge shows the engine warming up, reading near normal this whole time). I got within a mile of work and was stopped at a traffic light for a few moments and thought "what if I re-start?", so I did. Shut the engine off and restarted. Instant heat. Worked fine after that.
Over the weekend, I haven't let her warm up for 10-15 minutes, instead I've started her up and let her run for 3-5 minutes and then took off, heat works normally. Today, it was about 19 degrees outside this morning (warmer than it has been) and I started her up, warmed up for 10 minutes today, and drove to work, heat worked normally.

Coolant is full, tests good, no other issues. Any ideas why it would not heat up those two days until I stopped and restarted the engine? Thank you!

JS

Heater diverter door hanging up? I don't remember if it's electrical or vacuum in that year.

That's what I was thinking, blend door/diverter door hanging up due to extreme cold.

That should be covered by my MaxCare warranty, I believe.

JS

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.

Last week was damn cold for us here in Illinois.
My 96 Dakota which will normally roast me out of the cab was lukewarm one morning. Which I thought was weird.
Most of the drive is straight south about 20 miles, the wind was heavy out of the west
The last 7-8 miles is driving east. As soon as I made that left turn and headed east I had heat again when that wind was behind me instead of to my passenger side. I don't ever remember that truck being real drafty but that day as I drove south I had a pretty good draft out of the passenger door. But not since. Heat is back to a blast furnace

My truck sat outside exposed to the wind, the cold, and the snow from just after 2pm Thursday afternoon until around 9am Sat morning. When I opened the door, there were lots of little snow drifts inside the truck showing where air leaks were. I have a lot more drafts then I thought I had. I should have taken pictures, hopefully I'll remember where most of them were.

I checked the coolant (again) this morning while it was cold (sat overnight) and only added a few ounces and it was topped off (not very low). Tightened cap and started up, heat working perfectly today (although it was around 30 degrees this morning, last week it was -15). Heat roasted me on the way to work (super hot) so much so that I had to turn it down.

JS