What is that weird feeling you get when it feels like your stomach is floating inside you when you go down a steep ride like a roller coaster?

Gravity.

Your stomach is floating inside you, sort of. In freefall as the coaster rolls down, there's a feeling of weightlessness.

From Why does your stomach feel weird on a rollercoaster? | How It Works Magazine --

On Earth, gravity pulls us towards the ground. However, the force you notice isn’t the downward pull of gravity, but the upward force of the ground beneath you. The ground pushes up on your feet, pushing up on your bones and organs. The sinking feeling in your stomach when on a rollercoaster or driving over a hill is caused by a change in force experienced by your organs.

When in freefall, every part of you is accelerating at the same rate, which gives you a similar feeling to weightlessness. There is no upwards force from the ground to cause your organs to be compressed, so they are floating inside of you, even though you are falling. Our bodies aren’t used to this change in force on our organs, which causes messages to be sent in the brain that something is amiss.

I love that. And roller coasters too!