whenever i come to a stop my heater in my 03 wrx stops working and blows out cool air... it only blows out heat when im driving...

this happens when my engine is fully warmed up ...

does anyone know what the problem could be?

 

First step is to check your coolant level. If it's low this can happen. Check for leaks if it does turn out to be low, the coolant really shouldn't go anywhere.
There can also be air in the system, this usually happens after a cooling system service if the system wasn't "burped" properly. It's more of an issue on the 02-03's without the second cap on the radiator itself.
Less likely, but possible, is a blockage in the heater core. Normally this happens mostly on high mileage vehicles that have not had the proper cooling system service done resulting in corrosion and sludge buildup.