I think you must check your car and ensure that this thing happens sometime or on the regular basis. You just check it out.
No work on the coolant system while the engine is hot! Mini heater is not blow hot air when the engine is hot either the thermostat has gone or maybe it is faulty. You have a simple restriction that will be easily fixed by replacing defective hoses and/or flushing your heater core.
You can also cross check the operation of the heater valve units and when you do this, to see that it opens and shuts as it's supposed to. Physically manipulate the valve through its full range of motion and see if it works while you have the garden hose flushing out the heater.