This wasn't my imagination and it wasn't a coincidence. A quick Google search confirmed my suspicions: I was in the grips of a wicked case of MSG Sensitivity Syndrome.
I'm not usually one who suffers from syndromes (nor, for that matter, one who even believes in most syndromes), and I'm not a fussy eater. Still, I could scarcely have been more incapacitated if a 50-pound sack of MSG had dropped on me from the sky. Also, I'd eaten at this restaurant several times before and never experienced anything like this. That being the case, I assume that whoever prepared my food that night had dumped in a little bit too much of the stuff. Lucky me.
But what is MSG, anyway? What purpose does it serve in cooking and how does it do what it does? To answer those questions (and others), I put together this article that explains everything you need to know about MSG.