One possibility is hypoglycemia (low blood sugar).
Before I realized I was having low blood sugar episodes I could feel like I was having a mild seizure but only internally. I knew I had to eat but for a short time was so disoriented that I couldn't figure out how to eat even with food in front of me (parts of my brain had so little energy they could barely function). I would get shaky, have extra fatigue, stumble as I walked, slur my speech and couldn't speak at all sometimes. I would have anxiety and major irritability. If I tried to sleep through one of these episodes I would wake up feeling like I had brain damage for the rest of the day with extra fatigue. Eating regular small meals prevents these episodes.
Do those symptoms sound familiar to you?
If you have definitely have ME/CFS then living in the gym, or any kind of exercise past a certain threshold, will make you worse, possibly permanently. Many people here on PR have stories about exercising with ME/CFS and never recovering.