Why is my heater only blowing hot air

Why is my truck heater blowing hot air only?
Will not change over to blowing cooler air
when I change the temp control knob?

While driving around tonight in my truck with
the wife and dogs, the windows began to fog,
turned my heater on low, turned it to
windshield defroster setting cleared
everything perfectly ( it’s Maine and only
around 36 degrees outside) , however it began
to get to hot so I tried turning the temperature
down however the temperature would not turn
down, it just kept blowing the same amount of
warm air, I’d did not get cooler no matter what
I did, I turned my ac on and turned it to blow
on my face, but still no cold air, only Blowing
hot air, my blower motor controls work, all the
different blower selections work, but I cannot
get it to blow cooler air, anyone have any
ideas as to what is causing this?