That is what you would expect. With the A/C working, the condenser that sits in front of the radiator now gets the cool air first and heats it up before the radiator gets it. (This presumes your fans are running when the AC is on.) So, the radiator runs hotter, and you see that on your gauge. But, bumping up against the red is too hot. That would motivate me to start looking for cooling system problems, like low coolant, weak water pump, inefficient radiator, cooling system in need of a flush, new or lower temperature thermostat. That sort of stuff.
[edit: ha, PontiacDad, beat me to the punch!]