Why is my 2003 ford Taurus overheating when I use my air conditioner?

Why is my 2003 ford Taurus overheating when I use my air conditioner?

Check the fan radiator, does it turn on and stay on? If old it might run, but slower than when new.
With moderate pressure spray a hose over entire front of radiator release any dirt, bugs, debris trapped in the little vanes.
Replace thermostat, use a quality one.
When car is cold, start car, let it idle, turn heat to high, fan on medium. After 5 minutes start feeling top radiator hose, check every minute. It should start to feel warm and heat out of vents. Eventually getting hot, if not then water pump may be bad.
If not replaced, flush coolant. You need to get the EXACT right mix, use distilled water an spec antifreeze, I would just use dealer motorcraft. Find system capacity, drain and flush 3 times with distilled water.
Capacity / 2 = add that exact amount of coolant. Top off with distilled water. Burp system.

Could be several reasons.
One is that the AC condenser is in front of and preheats the radiator, and makes it get even hotter as the AC freon is compressed and heats up.
Second is that the AC takes power, so you have to give the engine more throttle.
But there are other more subtle possibilities, such as the AC should make the fan go on high speed, and if the relay or wiring for that were broken, you could end up with no fan instead of a high speed fan. So check if the fan by the radiator goes on when you turn on the AC.
Of course also check the belts, coolant level, etc.

Might be that you do to have enough radiator fluid. Check that before using the car in the mornings when it's not hot at all. Possibly you might also have a blown head gasket too