I need help w/ 2007 Pontiac G6 Overheating

Here's the backstory. I got this car from a used
dealer, no issues other than brakes and rotors
needed replacing. Drove the car for like 6 months.
Then out of nowhere it started overheating and
red-lined and it shut the engine off itself. So I let it
cool off and then drove it home and parked it. Well,
I still have not figured out the cause. I have
replaced the thermostat and I want to replace the
water pump but I dont want to waste my money if
it's something else. I'm trying to figure out a
solution as I can't get it to a mechanic. Other than
a coolant leak, because I can't find one, what could
be the problem. (Oil is normal and at a normal
level, car isn't losing coolant)