My earlier reply was that mine was caused by type IV food sensitivity. Recently I started reading "Plague" (about research showing that ME/CFS was caused by XMRV; now discredited) and read how many people got their ME/CFS following a vaccination. I kind of passed over that, because I hadn't had any vaccination, but then I realized that I had gotten a vaccination: tetanus booster. I tried to figure out when I had the vaccine, and while I'm not sure how far in advance of the type IV response, I do remember that it was in advance, and months rather than years. I think if it was only weeks, I would have linked the two.
So, if ME/CFS develops anything like the retrovirus hypothesis, it may be that I had latent ME/CFS, and the tetanus booster triggered it into full form, which in turn made my t-cell hyperactive and reacting to foods. It seems likely that the type IV sensitivity was due to dysbiosis, since it disappeared 2.5 years later after food poisoning, which would have drastically altered my microbiome.