Heater core may be munted which generally means a nasty bastard job replacing it. Or if you have a coolant leak near where the airduct intake is that could be your problem, sounds unlikely but it's happened before. Are you losing any coolant? Also, what is your vehicle?
the core in my old datto let go one night. whole cabin stunk and fogged up like a mongrel. not to mention the bloody hot fluid all over my feet. get it sorted as soon as you can.
hopefully its just a leak near the intake for the air ducts, replacing the core means lots of pulling out and putting back.