Air Conditioning on "Automatically"??

Seems like whenever I'm driving ('02 dubcab) and I have the heater control set on full-cold with the fan on, my AC automatically kicks on, but the lighted button for the AC does not illuminate. The air coming in is significantly colder than the outside air. Am I crazy or is my AC running on it's own?? I can turn on the AC by pressing the button, but the incoming air seems to stay the same cold temp.

Thanks,

Beave