Low coolant level in the radiator will do that. The water doesn't get high enough to reach the heater core unless the motor is revved up. You have to pull the radiator cap to see this (COLD engine). If you have a wrong or clogged radiator cap, or empty coolant recovery reservoir, or plugged reservoir hose, you can get this pretty easy. OR, the head gasket could be blown due to the overheats and the water is disappearing into the combustion chambers. Check for 1-2 VERY clean sparkplugs, or run a compression test. Or a combustion products test on the coolant.

If the head gasket is not already gone, it would be very smart to retorque the head bolts. Most any overheat will start them loosening up. One pretty sure indicator of a blown gasket is loose head bolts.

There are a couple of other causes for your problem, but these are the ones to check NOW. Or you'll be looking up the "replace the head gasket" post...