It's statistically more likely that you're sodium sensitive than that you're sensitive to glutamic acid. If you got a racing heart but no headache, that's an even stronger indicator: people who are salt sensitive experience palpitations when they ingest too much of it, but people who are sensitive to glutamates usually present with intense headaches.

Another question to ask yourself: do you get headaches when you eat blue cheese, parmesan, mushrooms, or tomatoes? Those are naturally intense sources of glutamic acid.

I don't think many people are arguing that it's impossible to be sensitive to glutamic acid (the case is stronger that nobody is specifically allergic to MSG, though). But there are people sensitive to all sorts of things, including very basic food inputs like salt. We don't dismiss people with shellfish allergies and say "but shellfish is perfectly safe!". But we also don't treat shellfish like hazmat because some people can't eat it.

Finally: the reason people call out the nocebo effect is that the placebo/nocebo effect is extremely and counterintuitively powerful. You'd expect people suffering from a nocebo effect to have a dismissive reaction to that explanation, because the effect plays tricks on our thinking.

(I upvoted you, by the way.)