That's interesting. Now that I think of it, the nausea feels exactly like what you get from ingesting the tannins in overbrewed black tea. I just assumed my nausea from drinking green tea and from L-Theanine (which I know that green tea contains in a non-negligible quantity). Maybe levels of tannins in green tea are higher than in black tea? In that case the nausea from L-Theanine may be unrelated. Maybe it's a combination of both.
I think you might be on the right track, but I drink their green tea now and I don't get the same effect. I also drink overbrewed Twinings english breakfast tea with no negative effect.
I got side effects from some medication I took until one day I was about to take them and then forgot, still got the side effects, and finally found I hadn't taken the meds after all.
At that time I concluded it was all nocebo and the side effects were almost completely gone from that day on.