Your cat could have a sensitive stomach for several reasons. Many domestic cats can develop an allergy or tolerance to ingredients in low-quality food, such as corn, soy, wheat and other fillers. If you've fed your cat the same food for years and suddenly switched it up, this could unbalance their gut microbiome and cause an upset stomach. Another cause for a sensitive stomach is if your cat's diet is high in fats, so opt for leaner animal proteins.
Stress is another common factor for sensitive stomachs in cats. If you've recently moved, added a new addition to the family or are away for a long time, the emotional distress can make them unwell.
Medication or eating something they shouldn't, such as a bug outside, a plant leaf, or some human foods, can also irritate their stomachs.