Sleeping on back bad thing?

Ok, I’m only 7 wks so I didn’t think this would be an issue yet but I ended up falling asleep on my back (not normal for me) and I stayed in that position all night (also not normal) but I ended up waking up because of this horrible pain in my left leg. It was like completely dead, I couldn’t move my left leg from my hip down, I had my husband help me roll onto my side and within minutes it was feeling better. Does anyone know what might have caused this? I don’t think my uterus is big enough to be pushing on anything yet.