This is often true for cars in good repair and working order. Turning on the AC either keeps the fan fully on, or turns on a second one.
Having the AC on generates more heat in the cooling pack. If the car is actually overheating, turn off the AC and turn on the heat.
I'm too scared to calculate the cost of gas to get there. Hopefully you were driving for work.
Edit how did you survive all that time with stop start on!?