Sounds like a heater-core leak. Check for moisture on the passenger compartment floor, usually on the passenger side of the car. There is usually also a sweet/pungent smell (anti-freeze) that is easily discernible. A leaking heater core will cause you car to loose anti-freeze, and radiator pressure, which will eventually contribute to your car over-heating.
Leaking heater core or line running to it
It has happened to me before when the heater core (usually located behind the dash-board on the passenger side) has sprung a leak, Because the heater core's is essentially a radiator used to capture heat that gets routed through the vents in your car, it will fill your car with steam via the vents when it leaks. They aren't very expensive (about $50) for the core and about one hour of labor plus new anti-freeze etc could bring the repair to about $150 or more. It is usually best to flush the cooling system while you're at it.