Over heating but AC wont turn off?

I have a 2005 Grand Prix GTP. I went to pick up my son from daycare the other day
and heard what sounded like my tire losing air at a rapid pace. I went around the front
of the car and noticed I could hear it better standing at the front of the hood. I waited a
minute and opened the hood to see coolant dripping down from the hood up near the
reservoir. I didn't drive the car again until today when I was going to pick my son up
from his dads house HOPING it couldn't over heat and I could just let it sit after that
until I can get it fixed. But as I was driving the temp gauge was continuing to rise so I
attempted to crank my heat full blast. instead of heat though all that came out was
freezing cold air. I had the temp up at 90 on my thermostat on the dash. The car
started to over heat and my display told me it was going to turn off the ac to help. So
the ac turned off and my car stopped making it's way into the red zone and started
cooling down pretty fast.
Have any idea what could cause this?