Sounds like you are actually quite low on coolant. Let the engine go stone cold and check the header tank, fill it up 2/3 of the way, then start the engine with the cap off. Turn on the heater to full blast. Eventually the thermostat should open and I bet the tank level will drop. Keep topping it up and keep it about 2/3 full. Try to bleed both the radiator upper nipple and the thermostat housing before putting the cap back on. Let the car cool overnight and check the tank again, top off if needed. Eventually it will all even out as long as your thermostat is actually opening (easy to tell by touching the radiator hose that goes into the otterstat pipe).