"WHAM!" for me too. I first felt like a flu was coming on (full-body aches, malaise), but I was surprised that it was gone the next day. I wondered if it was a reaction to some oranges I ate (rare for me). A week or so later, I had some orange juice (again, rare for me), and this time it felt like a full-blown flu. When that passed a few days later, I had some more orange juice. Felt fine the next day, so I thought it wasn't the juice. The next day, the flu symptoms suddenly flared up. I just happened to notice the clock was showing the same time that I had drunk the juice 48 hrs previously. After some research into allergies, I realized that I had suddenly developed a type IV delayed food sensitivity, first to oranges, then to most foods, plus a baseline of 'not feeling well'. I managed to stop the type IV reaction 2.5 years later after food poisoning, but still had symptoms flare up daily. It took me years to realize that it was no longer a type IV reaction, but a reaction to insulin following carb meals. It took another 10 years or so to realize that it was ME/CFS.
Oh, I had a tetanus booster some time (weeks? months?) before the 'WHAM!'. There's no way to prove it now, but I expect that was the trigger for my ME/CFS.