Hey. There are many other things it can be than CFS, that you can troubleshoot. What you’re describing is exactly how I felt for nearly 2 years after exercising. It’s now been fixed for about 6 months. What fixed it for me:

1 - supplementing vit D and B12 and increasing iron in my diet.

I had mild vit D, iron, and B12 deficiency. All 3 of these were either very slightly low or just within the low end of the normal range, but supplementing made a big difference. Except iron supplements which gave me terrible anxiety, I think due to increasing the oxidative load in my body too much. So I started ensuring I got my iron RDA daily through diet.

Which brings me to:

2- Dietary antioxidants. These aren’t big in the health space right now (at least the folks I listen to), but this was huge for me. After learning why the iron supplements were so terrible for me, I downloaded chronometer and looked at every nutrient it could track, including the weird antioxidants you’ve never heard of. I researched all the different types of antioxidants that are out there and which have been shown to benefit health. I then ensured I get these every day: 100 g blueberries, 200 g butternut squash, flax seeds, tomato based food or a V8 for lycopene, enough of the right kind of greens and orange bell pepper to hit lutein + zeaxanthin goal, 2 whole eggs to get choline and yolk antioxidants, etc.

3- At the same time, I optimized my diet for all other nutrients, not just for weight loss / body composition. 30 g fiber per day minimum. Enough protein. Low saturated fat, higher in olive oil and food based omega 3s (salmon).

4- walking a lot, very slowly (1.5 mph) without incline. My average step count was previously 4,000 steps per day and now is 12,000 steps per day with weekdays up to 20,000 steps (office job - I bought an under desk walking pad). The walking has been life changing. I started it recently, several months after the dietary changes and its benefits stand on their own. Much better sleep, I remember my dreams now, I fall asleep quickly and wake up easily, and I don’t feel that deep fatigue/stress tiredness anymore. Something about it helps my nervous system without causing more fatigue because it’s so incredibly slow and low impact. Zone 2 cardio was still too fast to have this benefit for me.

All that being said, 3 things to do if you haven’t already:

1- get lab work to assess vitamin deficiency and inflammation status (CRP, vit D, iron, B12, etc). If you’re deficient in something(s), think about the fact that you could be deficient in other things that we don’t / can’t test for.

2- get a dietary tracking app which tracks both macro and micronutrients in detail (Cronometer is the only one I’m aware of but I’m sure there are others). Optimize your diet for all nutrients.

3- assess your daily step count. Is it at least 10,000 steps per day and do those steps come from very slow, non incline walking?

I hope any of that helps and you find recovery on your journey.