Either the heater core is partially plugged or you have a vacuum problem causing your underdash ductwork doors to not open and close properly, or you might have a bad heater control valve (located in one of your heater hoses under the hood). Slim possibility that it is just low on coolant, this will sometimes cause that. Check and make sure the radiator is full, not just the plastic reservoir.
What Would Cause A Car Heater To Blow Cold Air When The Heat Gauge Reads Hot? This Is After Changing The Thermostat. And It Happens Mostly On The Highway.
Under hood ,,look for small valve located on heater hose may have small plastic vacuum line hooked to it.you can usually open it by hand to check it