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you didnt top off your coolant the last time it was drained/refilled.
an air bubble passes by the thermometer and gives a high temp since the air is superheated compared to the coolant. when you turn on the heater, the coolant has more area to cover with the same amount of fluid, so it exacerbates the problem.
cool the car down completely overnight with the heater set full red, then open the radiator cap and check your fluid level. you should be low. refill to top off the radiator and fill the reservoir 1/3 full. open the bleed valve at the rear of the motor, usually near the thermostat and let any air bubbles drain. yo know it's done when you get a steady stream of coolant running out. close the bleed valve and top off again as necessary. start the car with radiator cap off and let it idle til full temp with heater full blast and you see the fluid begin to circulate in the filler neck. if the level drops, add more fluid til full.
this should remove the air bubble in your cooling system.
you may need to replace your thermostat as the air bubble could have caused the wax pellet to die.
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