HELP! Heater problem

Ok here is the problem. This may be something simple but I am not for sure.

When ever I am driving my car will put out heat. The car has no problem warming up and producing heat while I am driving. The problem is when ever I come to a stop the heat quits blowing and it starts pushing cold air.

Does this mean I have a leak in a line some where? I have no clue.

Thanks,
Mark D

u need to turn the circulation buttom off