Were you doing water fasting? Have you been eating enough nutrients during your eating window?
If you weren’t doing water fasting, and instead were “dirty fasting” like drinking no calorie drinks, butter coffee, pre-workouts, stevia drinks, etc. If so-you were technically doing a low calorie diet and would not have had all the benefits and your body would react poorly, especially at your weight and health.
If you are not getting enough nutrients during your eating window, it could also be detrimental to your health as someone who is constantly using a lot of energy and stress to the body.
The only other option I believe could be the reason is, as you mentioned, you’re quite lean, and if you’re doing constant exercise, it would be a lot harder to get onto ketosis because you don’t have much fat to use as energy. - If this is the case, I highly recommend you keep a short fasting window only and try not to do any long fasts. If you enjoy doing long fasts, I would limit it only to 24 hours only once in a while. Also, with how much you must sweat as an athlete, you could also be missing electrolytes even though you are already taking them. After all you are using a lot of energy and would be sweating more than most people.
If you still don’t feel well, I would consider not fasting for maybe 3 days of the week (not even IF) and see how you feel then. You shouldn’t put on a massive amount of weight, but it might help with your fatigue.
Hope this helps, feel better!! -If not and you still feel this way after, I highly recommend consulting your doctor. All of this is only my opinion after looking into a lot of research, documentaries, and videos. Regardless, I’m still not qualified at all to diagnose you.