This response made me laugh. My girlfriend and I call these shudders “Vegan Orgasms.”
“Orgasm” because of what you mentioned, and “Vegan” because they come about with no beef/no fish.
Oh, I used to get these all the time when I held off going to the bathroom for a long while–like back in highschool, when I’d wait until the end of the day to go. On my personal list of pleasurable experiences, it is/was right up there with orgasm. 
Still have the experience from time to time, but nowhere near as frequently. Maybe I need to start holding it again. 
In general, I’m fairly prone to the mysterious shudders, and’ve always thought I was a bit of a freak for it–I get lots of odd looks and questions from friends when one hits me, and when I say nothing’s wrong, that just gets me another odd look.
The more cultured among us refer to this with a French accent, pronounced “Pay-Shiv-Airrr”
We don’t know. I mean, it’s inexplicable innit.
Heh, yeah, I guess you got me on that one.