just to add to bobs answer, we have seen many come in with bad intake gaskets leaking coolant into the oil. you can add dye to the coolant and when it shows up in the oil, the intake can be examined for the leak.
No, not a freeze plug. More likely to be a leaking head gasket or intake manifold allowing coolant into the oil. You may want to find a GM specialist. Here's a directory link for you: https://repairpal.com/directory?address=92084&car_brand_names=Pontiac