Yes, I have that feeling too. It is called the perineum- area between your testicles and anus... The pain seems to specifically reside around the area of the seminal vesicles and prostate, which can cause a stabbing or throbbing sensation deep in the pelvis around the anus and prostate. I believe there is acute nerve damage or structural problem in this area, which is directly correlated with the array of POIS symptoms we experience. It is very possible this is related to vagus nerve malfunction or auto-immunological response by the body which can unbalance most everything. What is certain is the body/mind symptoms are real and seem to be a secondary response by our bodies to an unknown impetus- which is where the problem resides (wherever or whatever that is). My guess is as good as any, but I would implore you guys to investigate acute damage or any kind of back troubles stemming from the sacrum; because it is the simplest solution to a complex problem. I am investigating all of these possibilities right now under doctors care.
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Btwn,
I have found ice and heat help with this acute pain. Although, I would stick to ice when you are very symptomatic and do both as you recover. Over months of trial and error I find icing actually helps with most POIS symptoms just in the area of the perineum meeting the anus... It sounds strange but swear it wks. Use an ice-pad for comfort. It's bizarre how well it wks. It only decreases symptoms temporarily though, so don't expect much else besides an hour or two of relief (which can mean a lot with POIS). Try it, because it is not invasive and will not effect your health in a negative way, it is easy. But give it a couple of tries before you quit.