robi123,
It's possible that there is some MSG in some spice mixture that they use. Or perhaps it's some other ingredient that you are reacting to (chili powder, for example). Does this happen only when you eat at this restaurant, or does it happen when you eat Mexican food in general?
I've given up eating in chain restaurants of all sorts (here in Germany) because of the MSG and other "enhancements" that are added to foods. Sigh.
--Rheanna