Here is an interesting article that might shed some light on these mysterious symptoms: https://hhv-6foundation.org/chronic-fatigue-syndrome/cfs-a-herpesvirus-infection-of-the-vagus-nerve
Could you shift this to the right place? I don't how how to shift an open discussion to another forum.
There is some immunomodulatory effects mediated by serotonin (https://www.hindawi.com/journals/jir/2015/354957/).
Medication working on serotonin receptors also alters immunofunction (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6509093/).
Possible underlying infections might get adapted to this new function. In sudden withdrawal immune system can't cope the viral (or pathogen) load and