Don't remember exactly where I read it, but the strength of your reaction to it can be tied very closely with the amount of sleep that you got the night before. I don't know if that's a function of cortisol or what, unfortunately the only decent cortisol test is a four-point saliva test that you have to collect yourself and send in for analysis.
Other than adrenals, I don't know what other hormones to even look at, this doesn't seem like a neurological thing local to the penis so much as it does something systemic out of whack. My cortisol was off and I've been targeting that lately, but I got an insane reaction from the PT (and I always sleep like a baby, moreso since I added dhea and pregnenolone)
As for filler changing sensitivity, it's really no different than squeezing your
Dick harder with your hand, if you have a bigger
Dick, it's a tighter vag, harder squeeze. It's also a high likelihood that tighter skin means greater stimulation of nerves near the surface.
