Heat Question

2001 Blazer Trailblazer 65000 miles.
Checked the radiator level yesterday and it was down a little bit (Put about a cup of coolant in). The puke tank was where I normally keep it - above the hot mark. This is the first it has ever been low. I check the puke tank any time I am under the hood and open the radiator cap at least once a month.
When I turn the heat on, it is strange. I have heat if I put is straight on defroster or on defrostor and floor. If I put the control to blow air out of the dash vents then it is cold!!!
Also today I checked the radiator level again and it was down about half a cup.
Does this sound like a thermostat problem? Might I be having problems with the blend door? I put a stant cap on at least a year ago. Also when looking in the radiator when it was low, it looks clean and shiny as a whistle.
All help greatly appreciated!!

change the radiator cap

Stant would be good choice.

lol didnt see that. You could check the thermostat too.There not that expensive to buy.

Get your system pressure tested to find out where it is leaking from. Do all of your vents work normally? Any hissing sounds behind the dash?

Update::

I have put a new stant cap on. Also today I put a new thermostat in.
Both hoses going to heater core are hot. When I move the temp control all
the way to the right on the read mark I have great heat in all positions. If I lower the temp - I still have heat in defrost and defrost/floor position, but other positions are cool.
Would this possibly be air in the heater core?? Question: to flush the heater core, I remove the two hoses and spray water first in one direction and then the other. Should I cover the distributor and other stuff directly below hose connections before I spray??

Any and all help and suggestions greatly appreciated!!!

It certainly wouldn't hurt. Did you ever find out where you were losing coolant?

No I did not. After the first two times it was low, it hasn't been
low since. Thought maybe the old cap was bad.