I always thought it was carbs too (ex-Atkins), but I don't get the same reaction from non-grain carbs, I'd just never tried it that way before my first Whole30, grains are pretty pervasive where I live, prior to Whole30 I don't think a single day of my life was wheat free (even my first baby cereal contained wheat and many Atkins & low carb products contain wheat & gluten).
I've also found it's a multiplier. If I had wheat + dairy + soy + seed oils, the result is extreme. Each one gives me inflammation, but to different degrees. Wheat is the worst, but dairy amplifies all the others (pretty sure it punches little holes in my guts and all the other rubbish just invades my body through the holes). However some dishes contain several items by default (eg. takeaway pizza) and these are things I have to completely cut from my life, there is no 20% flexibility for me. A little dairy I can tolerate from time to time, but it's critical I don't mix it with any other inflammatories.
A lot of doctors ignore inflammation "water weight", "fluid retention" etc.
It has serious implications for the body and should not be ignored, it's not just water, there are markers in your blood too and they are not good.
I have inflammation trouble with all of these: https://en.wikipedia.org/wiki/Prolamin
I lost an entire cup size giving up wheat, at first I was horrified, then the horror actually sunk in that it was really inflammation, not a real part of my body.