For the archives (and following up on my posts in rpdb and rpdh):

Cold tail (aka limber tail) is known in veterinary circles as post-
exertional coccygeal myopathy. It's accopanied by an elevation in a muscle
enzyme whose name I can't remember, but which indicates likely muscle
damage, even if only temporary.

One vet neurologist reports his own dog got it after having a bath in warm
water in a bathtub.

This info is courtesy of my vet internist friend who's doing a neurology
clinic at Ohio State University.

Cate