Depends on what kind of stomachache you would like!  :)

For instance - if you drank a a large quantity of milk, it would give you an INCREDIBLE stomachache...  but it would come with severe nausea.  You'd have to fight very hard not to throw up, and you'd have the sickening smell of milk in your throat.  Plus, you'd feel very cold.  Anyway, you'd be very gassy, with devastating cramps.

You could just eat a lot of hard-to-digest foods.  That would be pizza, cheeseburgers, steaks, chocolate brownies...  Eat a lot of all of those things, and you'll just get severe indigestion for a few hours, without the nausea.

 actually tried everything.... my stomachache only last for few hours LOL. the milk, I wouldn't dare tho hahaaha. I even eat a lot at the buffet... nah nothing works, i'm just the girl that can eat anything ;P I think I really enjoyed having stomach ache because it lets me poop. 

 wow hearing you saying it just gives me a stomach ache already haha