--opens up two other possibilites:

--neruomuscular compression, especially from the lower part of the spine (and that would hardly be a surprise, right?), or

--ciruclatory issues in the lower extremities.

This is, of course, in addition to the electrolyte possibilites.

I too get cramps during some of my stretching exercise work-outs, generally in the lower legs and toes, and they are associated with certain positions I assume--usually when I am tightening muscles or stretching them considerably; it seems that the conscious process of "hardening" them produces spasm from time to time, though it goes away within a minute or so if I release the area. I have found these have lessened with good hydration and major magnesium citrate dosing, though they haven't gone completely.

The fact that you are getting sensory symptoms with these sort of points to some sort of compressive forces--could be the pressure of muscles/fascia on the lower spine or cauda equina area when you're lying on them--could you lie down on your side to sleep? (And how was your night?)