your cooling system is for the engine but you also have a pipe running behind your dash into a heater matrix and then back into your engine bay.
your fan for your heaters in the car blows air through your matrix which when the engine and water is warm it blows hot air into your car.
you may have sufficient water to keep engine temp down but its air locked the heater matrix.
check water and top up if required, keep cap off and run engine squeeze all the pipes and top up as required. that should get all the air out and make your heaters stay warm