That is not common, but I have heard of caffeine, even low dose causing nausea.

However, the tannins in the tea might be causing your nausea.

I will sometimes get nausea from Tea or Coffee. In order to prevent the nausea, I make sure to eat something along with the Tea or Coffee that goes well with it. I don't know what you prefer, but try out different foods, snacks, or something that appeals to you along with green tea.

Green tea contains polyphenols that may potentially cause nausea. Adding a little milk blocks some of them.

If I were you, I would attempt to deduce if you may be allergic to something in the Green Tea if the milk and eating food with tea techniques do not work.

1. Caffeine

2. Tannins

3. Polyphenols like Catechin

It could be a combo of each even. Deduce!