I get that too, even to the point of falling off the chair and not breathing. I get it also in my neck, unable to lift my head sometimes.
I think it's just extreme fatigue, although this idea that it's minerals or something makes sense too.
I don't know for sure what's going on.
EDIT: By far, my diet is not the greatest at all, so it could be some vitamin deficiency too.
I only eat chicken, chicken broth, occasional egg yolk, magnesium citrate, and tiny amounts of celtic Sea Salt and purple dulse.
For a while it was just straight chicken, period.
How's your diet?
I'm sure you eat more variety than me, and I'm guessing if you're diet is restricted in some way right now that you started having this problem before you started cutting down though, so perhaps it's not purely dietary in origin, but maybe it's a factor.