So I replaced the radiator a few months ago and put in new coolant. After a few initial system burps, everything was working fine.
Recently, as the system pressurizes, the low coolant light comes on. When I stop the car, and remove the reservoir cap, the expansion tank fills up, like it's over pressurized.
I'm not losing coolant, none in oil, no drips, etc. Is this just an issue with the reservoir cap or a thermostat issue?

Thanks!

 

Does anyone have any idea why this is happening

 

it's possible when the car was driven low on coolant or overheated (I know it never did to your knowledge, but you probably replaced the radiator because it was leaking) the head gasket was damaged. Extreme pressure build up is a symptom of that. Borrow a head gasket leak detector kit from your local auto parts and check for HC in the cooling system.