Loster
Hi there mate,
I do relate to this. Been trying to kill off a fungal infection and probably dysbiosis for years.
At my worst with it I was the same as you describe here - eating almost anything left me feeling lethargic and brain foggy. It was very disturbing, and confusing.
The answer to why it's so immediate is that you could have esophagal candidiasis. It could be there lining the the esophagas and the fungus may respond straight away to the influx of food (you don't get different responses from carb-y meals to non-carb-y ones?). And of course when fungus feeds, like any organism, it gives off waste products, and of course it attaches itself to your gut lining and can even digest parts of it and poke through.
Its a pernicious and horrible creature!
That's just one theory about why the quick reaction. There could be other explanations. But if it is either a fungal or bacterial overgrowth (which is what SIBO is), the treatments will be the same.
Your next step may be to get it tested. I got a test done with Great Plains Labs called "Organic Acids Test + Microbiology Test" (I can't post a link)
It isn't cheap, but they are very professional, and it was very much worth it as it did establish a fungal infection, and some bacterial dysbiosis. Getting that objective proof can give you a path to pursue and ease the mind.
I feel for you mate. It's really hard having these symptoms, but get it diagnoses asap, then you can start on a regime which bolsters your immune system, sends in antifungals and probiotics, and there some supplements that can escort out the fungal toxic byproducts and some nutrients you can take to help your body detox from it.
Give me a private msg if you wanna chat further.