You can say: "My tummy hurts." 我肚子疼。 “Don't show your tummy." But "tummy" is more of a cute word and is directed at children or used by them. Generally adults don't use "tummy" very much.

You could use the word "abdomen."

口语的话可以说 suck in your belly, suck in your stomach 都可以。比如拍照的时候，提醒别人:"sick it in!" 那样拍照显得身材更好。：）