I have a '06 Maxima. This is the 1st winter I noticed this... while Im driving the car heats up pretty quickly and gets nice and warm... yet, if I park or I'm sitting on the break for a few minutes... only cool air comes out... what's wrong with my heat? Can I fix this? Burrr...

A couple of things could be contributing here: If the thermostat is stuck partially open, the coolant tempurature will range normal while you're driving but once the engine load is taken away, things cool off quickly.
Another possibility is a cooling fan staying on.
Check that the coolant is clean and full. If the cooling fans are running when they shouldn't be, you may have a sticking relay or a faulty control head for your HVAC system.