Some people are more likely to have inflammation, allergies, pain in general. Due to my height I started having back pains very early, around 16 years of age. I believe that the POIS is connected with some chronic process (in my case, back problems).
I always played soccer and did swimming. After the birth of my daughter I left the sports because I was tired, sleeping badly and came the problems of marriage too. My body became weak, my mind became sick and so my POIS came. Each had something like respiratory allergies, skin allergies, bowel problems. Maybe before POIS these problems were less intense because there was no chronic vicious cycle. For me it is clear the connection of my POIS with my problems in the thoracic and cervical spine. These days I only have 10 to 20% of the symptoms because I treat my spine every day with exercise. I have to dedicate 1 hour a day or more to maintain the health of my spine and in this way the symptoms are less intense. It also has the fact that the person who has chronic inflammation ends up by lowering the neurotransmitters of well being which ends up affecting in some way the brain chemistry.