Flat on my back

So, I've wondered about this "can't lie on your back" thing. I've been lying on the couch and haven't noticed anything. (And worrying that I'm really doing some secret damage and can't tell.) Last night, I'm vaguely aware that I'm dreaming about lying on my back, and seconds later, I wake up, and sure enough! Flat on my back. So I roll over and go back to sleep. I woke up with the vague recollection of all this and figured it was all just silliness. But then today, at the doctor's visit, I was lying on my back and they were checking everything out and I was getting crazy dizzy. And dizzier. I had to ask to roll over. So, I have learned that: 1) you really shouldn't lie on your back for a long time, and 2) that my body (or the baby) won't let me do it.