[Baldwin]

Could be the tannins in the green tea; they are most commonly associated with items like red wine and chocolate. While there's no doubt tannins can trigger migraines, the mechanism is pure theory at this time---while having anti-oxidant properties, they also act as anti-nutrients (like phytic acid), and can bind to nutrients as well as to the GI tract itself. In the case of migraines, they are thought to interfere with the utilization of serotonin.
I'd suggest trying both black and white tea; white is less fermented than green; while the actual level of catechins will be higher than green, the fermentation/oxidation process may be creating compounds that are triggering your headache.
On the opposite end of the spectrum, also try black tea; it has less catechin than either white or green.