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This "falling" sensation is likely due to what is known as a "vagal response". When you develop pain inside your body or nausea, this can cause a drop in your blood pressure and blood sugar, leading to this vagal response. Once your BP and sugar returns to normal, this sensation goes away. Hope this helps.
Are you asking why you get queasy in the stomach?
This vagal response occurs due to the queasy feeling you are getting. I cannot determine why you are getting queasy. It could be a GI bug or possibly a stomach ulcer that your doctor may need to evaluate for.