Curious about the relationship between temp and pressure.

Messing about with sterling engines recently and came across this trying to pressurize the room with ice.

The room's pressure was higher the colder it was, and when the sterlings heated it up again it dropped significantly down to almost nothing.

I was under the assumption that an increase in temperature also caused an increase in pressure, yet the opposite is happening here. Makes it convenient keeping the room from exploding though, so not complaining too much.