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