From Longman Dictionary of Contemporary Englishupset stomachupset stomachMIan illness that affects the stomach and makes you feel sick → upset
Examples from the Corpus
upset stomach• Wainwright will play despite having returned from representing the Barbarians in the Hong Kong Sevens suffering from an upset stomach.• He missed the 2-2 draw with Boro because of an upset stomach. and reserve Iain Jenkins is standing-by.• Backache, headache, insomnia, upset stomach, irritability. 