Frozen Taco!

Do you have a scan gauge or ultra gauge? That way you have an idea of what your engine temp is, you could be low on coolant, thermostat stuck open, or heater core clogged, or air in the system. You can always try flushing it out, (preventative maintenance) but if you have over 100k and haven't flushed, I'd say go through and make sure you do a flush and fill, replace thermostat. I just replaced my water pump even though it wasn't bad, but I'm at 160k. I found out that you have to burp the system (get the air out) or else you won't get any heat.

 

I just had this same issue and found My coolant was low, over 1/2 gal low. After filling it back up and a short drive the coolant was again low.
I Removed my skid plates and found that a lot of anti freeze had collected on top of them but hadn't leaked on to the ground yet and I couldn't find where the leak was coming from so I had it towed to the nearest dealer. They found that a wield on the bottom of the radiator had crack. Service rep informed me that I was the 4th radiator that his shop has replaced.