因为stomach的词尾ch发音是[k]而不是[tʃ]。

His stomach bulged after the dinner .他吃过饭以后，肚子都鼓起来了。A blow to the stomach made him curl up .他腹部受到一击而直不起腰来。I have a sharp stab of pain in the stomach ..我的胃突然感觉一阵剧痛。She injured her stomach by eating too much .她吃得太多伤了胃了。She injured her stomach by eating too much .她吃得太多，伤了胃。

ch 结尾的名词一般是加 -es 构成复数形式的，这是由于受它发音的影响， 而 stomach 的 ch 发音比较特殊，所以它的复数形式也是特殊的，就是直接加 -s 构成，即：stomachs