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.