Take this medication 'on an empty stomach'. [=when your stomach is empty because you have not eaten for a time]
When I had the flu, just the thought of eating made me 'sick to my stomach'. [='nauseous']
She felt a flutter in 'the pit of her stomach' when he walked through the door.
(..) l mean, it'll just go right through you. Sure! Let's all throw books at Myrtle because she can't feel it. Ten points if you get it through her stomach! Fifty points if it goes through her head! -But who threw it at you, anyway? -l don't know. l didn't see them. l was just sitting in the U-bend, (..)2002 Harry Potter and the Chamber of Secrets