1) Portman, Shanghai
2) Shangri-La, Makati
3) Japanese place in BJ on NE corner of Guomao Bridge
4) Sheraton, HK
5) Westin, Shanghai
6) Legal Seafoods, Boston
7) Salty's, Portland

All seven of the aforementioned places have caused me pain similar to that described in the OP. Because #6 has an exceptional record on the QC front, I'm inclined to conclude that I'm simply allergic to raw shellfish.

While I still do brunches, I steer clear of oysters these days. BTW, #1 was especially nice to me. They paid for my medical bills and invited me back to brunch for free!