Our bodies are horrible when it comes to spatial awareness.

Pilots are trained to ignore sensory input like the feel of gravity, the feel of being in a dive/climb/bank and use the instruments to make determinations about what the aircraft is doing. Usually those feelings are dead wrong.

It's amazing how flying into a cloud bank, even though you've been flying straight and level, suddenly your body screams at you that you're in a descending bank or something. You just learn to disregard those senses and trust the instruments.

As far as looking out the window, it was cloudy, at night, and at high altitude. At high altitude in straight and level flight your nose is pointed at blue sky, not the visual horizon.