"The answer is that no one knows with absolute certainty,” said Dr. Brad Sagura, a surgeon at U of MN’s Amplatz Children’s Hospital.
“There’s a network of nerves within the body, messaging between the spinal cord, the brain and other structures. When you hit the peak of a coaster and drop quickly, things inside start to shift around. The liver & spleen are relatively secured by suspensory ligaments,” Sagura said. “But the intestines themselves are relatively mobile. While your body is secured by your seat belt, the organs are free to move about by some extent. That contributes to the free-fall floating sensation.”
The movement isn’t only the movement of the organs, it is also the movement of what’s inside them. “The intestines,stomach, bladder, hold liquid,” Sagura said. The fluid within those structures probably plays a role in that sudden drop.”
There’s no long-term danger from your organs slightly shifting around. They go back to where they started. But the movement is enough for your nerves to notice that something’s happening.
I think it is because of the tension and motion on the coaster that causes a gravity change. The coaster pulls your body against the gravity when falling, which gives the body a mixed direction reaction.
When you're in a car and take a right or left hand turn your body moves to keep itself straight up, even though you feel like you're moving to one side. On a roller coaster you're stomach is in one place high above. When the coaster goes down your body adjusts to that making it feel as if your stomach feel like it just dropped.