I just thought I'd toss this out there because it's a rare disorder and someone who can't figure out what the heck is wrong with them might look here.

Myotonia congenita is a neuromuscular disorder of the skeletal muscle chloride ion channel, which results in a delayed relaxation of the muscles after voluntary contraction.

So if your muscles get "stuck" for a few seconds after you contract them, you might want to consider the disorder.

Another interesting trait of the disorder is the "warm up" effect. The more the individual moves, the easier it is to move, until the individual rests again. This means the individual can have difficulty moving one minute, and be fine the next.

"Fainting goats" have this disorder.