wow, that's interesting. I was dreaming one time, can't remember what the dream was about, but i do remember i had a lot of pain in my knee as if i had broken it or something, when i woke up a few hours later i still had the feeling and it turns out i had slammed against the wood that holds up my bed (my bed collapsed so the wood is right next to me), and it had sliced my knee in two places. I still have scars. Maybe you were hitting something against your lower back in your sleep.