[plaguuuuuu]

MSG is often used as a food additive in combination with heroic amounts of fat and sugar which would otherwise be unpalatable without it - particularly in a Chinese restaurant setting which is what people associate with MSG...

You're probably experiencing a sugar crash.

I suspect you don't experience the same after eating a bag of Doritos but they're absolutely loaded with MSG.. plus in all likelihood also sodium inosinate, sodium guanylate and who knows what else.