check it this way: Check that when you turn your AC on, does the fan for the condenser work? Is the radiator fan working when your AC is on? And running good?
then, the radiator.. open the cap up, is rust there? too much? how does your coolant look?
Try flushing the radiator, use the 'cheap flush' first, just drain all of the coolant from the radiator, fill it with water, drain it again, fill it up, then check if your car overheats as badly / quickly as it used to do..
If you feel that the behavior has been changed, get your radiator serviced.. if not, it may also be your radiator's cap thats not holding the pressure well. When the radiator cap isn't working properly, it will boil the coolant / water.. and you'd be able to hear it boiling specially when you turn your car off, do you hear that kinda sound?
Did you recently get the AC serviced / repaired?