why does the heat only works when i accelerate?

the 19th of february i parked my car and then realized there was antifreeze leaking out the bottom of the car. opened the hood and saw that the coolant liquid (green) was all the way to the top of the reservoir tank. So it has overflown. Let it cool off and came back few hours later to see if there was any leaks, but i did not find anything. Took the car for a spin, while the engine was warm enough i cranked up the heat and noticed it only blows hot air when accelerating. The air flow would quickly change to cold while on a red light.