I think you are on to something. If I got out to eat at pretty much any fast casual restaurant (chipotle, cava, etc) my fingers swell up the next day, I am insanely thirsty, have gained 2 pounds, and feel very unwell, almost like a hangover. 48 hours later I am back to my normal weight and feel fine.
I hate to be that person who's like, "I cook at home for health" but whatever they are putting in the food now, I cannot stomach it.