Robert Bruce and others report that the cause of fatigue while doing energy work lies in the fact that because our energy system may not be that fit as we begin, we spend more energy cleaning the 'plumps' than we actually store... and that's true... but there is more to it. It's sometimes the case that our energy 'plumps' are quite good but we are just making exaggerated mental effort to raise energy. That's what I was doing and I was feeling the energy but it's better to feel less energy doing it in a more relaxed state and at the end feel re-energized.
Beware as the symptoms of exhaustion, emotional crisis, etc are very real... I had them until I realized I shouldn't make this load of mental effort to raise the energy.
I don't feel anything if I don't put a reasonable degree of mental effort in personally.