2002 Buick Rendezvous Radiator overflows when cap removed o

I recently had the water pump replaced, and since then I can't remove the radiator cap when the engine temperature is cold without it overflowing antifreeze. The radiator is staying pressurized and after it overflows about a gallon or so of the fluid, I then can't add fluid to the radiator, only to the overflow reservoir.
Any help would be greatly appreciated.
Thank you.