Pressure build up in a cold radiator

There seems to be a very high pressure build in my radiator. I noticed this when I opened my radiator cap after parking overnight when the engine is cold. I've also noticed a slight increase in the coolant level in the storage bottle.

Is this normal or not?

Also once I start the engine without the cap replaced i see the coolant fluid gradually rising, over flowing as the engine warms up.

I was asked to replace the radiator cap and check but the problem still exists.

Thanks. Nev