I believe your heart skips a beat. A lot of people thing it's their stomaches.. not sure if I agree or not.

I think it's your center of gravity shifting momentarily.

That's zero Gs. When you have a hyperbola route like a road on a hill, when moving across the top part, you briefly experiences zero Gs. This happens with all things like this, mainly when you come to a stop. Such is the excitement factor with amusement park rides.

The feeling over a quick hill is a negative G force. You become (partially) weightless because as you go down hill the reaction from the ground pushing up becomes reduced and so you are in momentary free fall, similar to when astronauts orbit the Earth and become weightless. Since it is over so quick so don't get time to adjust and so the feeling is quite strange. The same occurs when you start to descend in a rollercoaster or very fast elevator or experience tubulnece in a plane that forces the plane down quickly. The feeling when you stop in an elevator is similar. When the elevator starts, you have a slight feeling of increased weight as the reaction of the floor pushing up is greater than the force of gravity pushing down because the lift is accelerating upwards and so the total force upwards is greater than the total force downwards. When the forces become balanced, you ascend at a constnt speed, but as the elevator stops, the force downwards (gravity, basically unchanged) becomes greater than the force pushing up, because the acceleration pulling hte elevator up lessens, allowing the elevator to stop. So since the forces are no longer balanced, again you experience a moment of reduced weight.

It is your internal organs shifting slightly upwards, due to the brief negative gravity force. As a child, I used to have reoccurring dreams that incorporated this sensation.