I've been going through a million tests lately at the docs. I have the same issue, about 20 minutes after eating (particularly grains) I get super tired and often fall asleep. It's like popping benedryl.
Although the exact reason may still be unknown for me, I did find out I'm anaemic and have an abdomen full of fibroid tumors, some which seem to be rather large. Although I did try a low-carb diet thinking it was blood sugar related, and even gave intermittent fasting a go for 3 long unpleasant months -- it did not solve my issue at all (and made everything worse, which is a long story). After taking iron supplements I can now safely eat a gluten free pumpkin whole grain waffle with impunity and am really enjoying that.
I suspect I have a low blood volume (POTS) along with anemia and when I would eat the carbo-super-rama foods, even though I eat complex and not simple carbs, that my blood was going to my stomach.
This doesn't totally explain it, I mean I walk around and blood goes to my legs and I don't fall asleep. But as much as I can figure it does have to do with iron, at least for myself.
Everyone here pretty much seems to heartily and enthusiastically endorse Keto and Low-Carb (sometimes I think a little too much so, to the point that any other option or a persons choice is dismissed out of hand as 'wrong'), so I suspect you'll have a cascade of answers of, "beware the evil carbohydrate" -- but I have had zero luck controlling anything about CFS with diet, although I do enjoy eating Vegan as a personal choice, (critters, environment + health reasons) and while being Vegan managed to drop my blood sugar way down into a nice healthy range and my blood pressure is the best I've ever seen it so I have chosen that this really works for me and I'm GTG.
Oddly enough my ferritin test wasn't terrible, but I guess I still needed iron. I also started watching out for calcium which blocks iron absoprtion.
Hope this helps. Please no one attack me for saying complex carbohydrates aren't the devil. I stress easily and dog-piling is very impolite. Thanks in advance for being civil.