I felt pretty normal for a couple weeks between getting a flu and developing my first severe symptoms of ME (PEM in my case).
If you don't react at all to exercise, then it might not me ME/CFS at all. But sometimes the reaction might be milder and more delayed, which can make it hard to pinpoint. Even before getting very sick, I would consistantly have the "flu" if I started any exercise program, but it was typically delayed by a day or two. The pattern makes sense in retrospect, in the framework of ME, but just seemed like bad luck with getting the flu/colds at the time.
It's also possible that you your fatigue and cognitive problems are solely caused by orthostatic intolerance, which can be difficult to notice as a specific cause unless you're looking for it. Do you feel worse when standing/sitting up, or after standing up for a while, especially if standing still such as in the line/queue at a shop? Do you feel better after laying down for an hour or so, even if you don't fall asleep?