From Harry Potter and the Philosopher's Stone

(After Harry had lost Gryffindor 50 points for the House Cup), Harry felt as though the bottom had dropped out of his stomach.

I only found "the bottom drops out of the market" which has nothing to do with it. It feels like the same as "Harry felt his heart sinked". Then why bottom drops out of stomach, it just doesn't make sense. Can explain it?

When you go up in an elevator that is rapid, the bottom feels as though it has dropped out of your stomach. But in this context, Rowling is using it to say that Harry P is deeply disappointed and rather apprehensive.
Well, maybe it was meant that his stomach turned upside-down.
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 Feebs11's reply was promoted to an answer.