Violent Shakes During WILDs

I've attempted to WILD many times, without any success, from weeks, both in afternoon naps and during the night.
However two or three times, when I attemped the WILD during afternoon naps, even before I could enter sleep paralysis, I got an incredibly violent body shake, as if I was shivering, but I literally moved every part of my body of about 30cm.
It lasted for two or three seconds.

Any idea what was that? I doubt it was any sort of vibrations because it was too early (and that wasn't a feeling), that wasn't some cold shivering neither, because I was warm. Uhuh?