Green tea contains a substance called Tannin. Some people are sensitive to tannin. Tannin sensitivity can result in an irritated stomach and feelings of sickness.
Drinking on empty stomach
Tannin sensitivity can be reduced if you ate something before or while drinking green tea.
The stomach produces more acid
Green tea contains Tannin which can stimulate the stomach to produce more acid. If you have an acid reflux then Green tea can lead to nausea.
Green tea contains Caffeine which can in some cases lead to acid reflux.
Theobromide and theophyliine cause nausea
Green tea contains theobromide and theophyliine. Both substances can cause nausea and can lead to vomiting. (See Why eggs make some people feel sick?)
Vitamin C can upset stomach
Green tea contains vitamin C. For some people vitamin C can result in an upset stomach and so feelings of sickness.
Polyphenols can cause nausea
Green tea contains polyphenols which is a compound that can cause nausea.
Tea can stimulate the production of gastric acid which can result in indigestion. Indigestion can make a person feel sick or uncomfortable.
While green tea is healthy still it contains substances that might cause some of the symptoms described in this article if they were over consumed. (See How your mind can make you sick?)
Why Green tea makes me feel sick?
Why do I feel nauseous after drinking green tea? Do you feel sick after drinking green tea?