Overly simplified explanation of Mind-Body Connection
Pituitary gland (“Master Gland”) communicates with the Hypothalamus in the Limbic system of the brain and is involved in secretion of neurotransmitters and hormones
Hypothesis is that any internal or external event may trigger the brain and/or body to alter it’s current or even later set points.
There is often a proceeding priming of hormones (maybe earlier that day) that predisposes sensitivity to stimuli.
Please search Psychoneuroimmunology and Psychoneuroendocrinology for more graphics and details.