Learn English – Word, idiom or expression to describe feeling full (after eating)

The sensation you describe occurs when you're in free fall, as described on HowStuffWorks:

But in the "free-fall" state, there is hardly any net force acting on you. In this case, the various pieces of your body are not pushing on each other as much. They are all, essentially, weightless, each falling individually inside your body. This is what gives you that unique sinking feeling in your stomach.

Unfortunately, there doesn't seem to be a specific term for it. (Unless there's a really technical medical term that involves your vestibular system somehow.)

On the other hand, that falling sensation (and accompanying twitch) you sometimes get when you're falling asleep? That's a hypnagogic myoclonic twitch.

The original post itself alludes to two perfectly good adjectives: intuitive and instinctive.

Googling "define intuitive" yields as the primary definition: Using or based on what one feels to be true even without conscious reasoning; instinctive.

"define instinctive": Relating to or prompted by instinct; apparently unconscious or automatic: "an instinctive distaste for conflict."

My sense is that intuitive implies some degree of certainty, even though one might not be able to present logical arguments that would convince someone else--or, at least, without more effort than one wishes to invest. Instinctive, on the other hand, seems to convey less reflection.

One might intuitively select pieces that will coordinate well when decorating a room.

One might instinctively react when one's child starts to walk toward a busy street.