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.
(..) Hey, you, get your damn hands off her! - You really think I ought to swear? - Definitely. Goddamn it, George, swear. Okay. So now you come up. You punch me in the stomach. I'm out for the count, right? And you and Lorraine live happily ever after. You make it sound so easy. (..)1985 Back to the Future