Thank you, Muon! I created this poll because a few days ago I read two threads in here, started by two different people, who said they believed antibiotics had cured their POIS. Now, in that thread you linked, I see that for some people POIS started after taking antibiotics. This leads me to think this: what if POIS was caused by a harmful bacteria that entered our bodies and is disrupting processes in us? Sometimes staying dormant, and flaring up when our bodies are under physiological stress?
Taking high doses of an antibiotic could damage our normal gut bacteria, or even bacteria that live in other parts of the body, some of which act as a barrier to prevent harmful bacteria from attacking the body. I'm quite sure that my POIS started because of the long period of colitis that I had due to my undetected soy intolerance. My gut bacteria were surely damaged or temporarily wiped out from the colitis, just as it would happen in someone taking antibiotics in high doses and/or during a long period of time.
That could maybe explain our muscle shaking (I think I remember you said you had this symptom too), if the transmission of nerve signals is somehow disrupted, or maybe it's some metabolic process.
Maybe this idea is stupid, I don't have the knowledge to know if it makes sense or if it's possible for such a thing to happen, but I thought I should share it.