Engine Cooling problem
2005 Buick Allure 6 cyl Front Wheel Drive Automatic 85000 miles

I bought a used 2005 Allure 2 months ago. It was fine for 2 mts. Last week an alarm came in while driving
"AC off for Engine Protection".
I read the manual - reset and added coolant the next day.
The alarm comes in each 10 or 15 kms now followed by 2 other alarms:
"Coolant HOT"
and then
"Reduce speed and pull off road asap".
What is going on ?
Please help....