my car started to overheat?

[Jons_G35_sdn]

last night after visiting a couple of stores i was headed home and noticed the temp gauge going up little by little and finally had to pull over at one point cause it was really high. I let the car sit for about five minutes then turned the heater on full turned the car on and drove temp whent back to normal and didnt go up anymore, only if i turn the heater off then it starts going up again what could it be? Also the electric fan that comes on when the a/c is on was on last night as soon as i start the car even though the a/c is off? Its a 2003 G35 sedan completely stock. TIA

[fixer]

thermostat is sticking, or you are low on coolant. if eng is low on coolant be sure to bleed the system after filling.

[jwerty1]

low on coolant, bad fan, bad temp gauge, the lists goes one, we need more info