I have a similar issue with peanuts, actually. I can't eat raw peanuts, under-roasted ones or "natural" peanut butter without having some discomfort (but nothing like this!) Sufficiently roasted peanuts or Jif I have no problems with. (You can all hang me now for eating Jif.)

The only soy milk-based drink I've had was the Bolthouse Farms vanilla chai stuff, and I've stayed away from drinking soy milk since then--again, that was the only thing I could think of. I don't know what brand Starbucks uses--I want to say Silk, but I don't know for sure.

Malkin--that was my mom's initial suspicion, but since it doesn't happen every time I get cold drinks, and the last time it happened I used my own cup, it's unlikely.