The products that the restaurant buys and uses, may have been pretreated with glutamate. So it may be "true" they didn't ADD any, but some may have already been there.

And another possibility is histamine. This is produced by certain organisms on stored food. Fish is a prime example. 2 summers ago my son and I were histamined by frozen perch. It didn't taste old or spoiled, but stored it would still grow some organisms.
By 3 pm both of us had burning feet. He had to take his socks and shoes off. This was new to him, but I just endured mine.
This attack responded to thiamine (which I had up there)... so if this happens again, try the B1...it might help (or not depending).
I suppose it could have been MSG too... a small local eatery in a small local rural setting.

Soy sauces are huge MSG containing products. I never use them, for that reason. If you order a soup of any kind, they usually are high MSG donators also.

People don't realize that bacteria continue to grow even when food is refrigerated or frozen. I am very leery now of ice cream following the long term Listeria contamination at Blue Bell in Tx.
Infecting people since 2013! We had a Listeria event in our state years ago--- a popular meat supplier. It was found that condensation from a pipe above a packing line, was dripping on the food! So I have no frozen ice cream or other treats here now...it is not unreasonable that we have probably been infected at least once. Listeria continues to grow in the freezer on whatever food it is resting upon.

Eating out is very hazardous. One restaurant upNorth, even has a disclaimer on its menu----"not responsible for food borne illnesses!"