As I was falling asleep last night, I paid attention to what position I was in. My natural, comfortable sleeping position is to lie on my stomach with my head turned to the right. When I do this, I curl my right leg up a bit, but my left leg is extended. So, I think this might very well be the cause (or at least the trigger) of my muscle pains in my left leg.
I have been known to fall asleep on my back, but I pretty much have to force myself to do it. I usually only do that if my neck is bothering me enough that that is the only comfortable position.