My heater tends to cool as I drive

My heater tends to cool as I drive. When I start the car it seems to warm the car and put out heated air. After I drove for 20 minutes this morning it got cooler and cooler. Last winter, we noticed that turning on the heat in the back seat didn't warm the car but put out cool air. We no longer use that feature. Could you please provide some ideas of what may be wrong?