I had very bad PN 4 1/2 years ago. I was also very very tired and thought I may have CFS.

My thoughts now:
I had back surgery 8 years ago. No other health problems at the time.For months afterwards I was completely exhausted to the point of collapse.

Fastforward to when I came down with severe PN. My body was a train reck. I think that when our bodies are damaged, the healing process requires extra energy/life force which is needed to repair/restore the damage. I think this causes that feeling of fatigue. The greater the damage the greater the recovery period and the more resources/energy needed by the body to repair.