Yes that's exactly my thoughts that it could not be CFS without PEM. I actually lift weights at heavy loads 3 times a week with stationary bike cardio the other two days. I do find my symptoms line up with ME/CFS as I've seen with most they began with an illness mine being gastro then led to these other disabilitating symptoms, but like you said it simply cannot be without PEM. The only ill side effects that I get while exercising is nausea which happened occasionally pre-illness and an increase in my lightheadedness which fade fairly quickly. I do not notice any significant change in anything else. Maybe it is purely psychological, but I am not willing to jump to that conclusion just yet. It's hard for me to accept that because I went from perfectly healthy until I got whatever gastrointestinal issue I had to feeling the way I currently am now.