I suppose if my vena cana was really being squeezed, I would not be comfortable!
You would know it. I had to lie on my back once a week just for an ultrasound with Harry near the end of the pregnancy, and I would get light headed, my concentration would start drifting and then start feeling extremely ill. From there my HG would take over and any chances of eating were completely shot for the rest of the day, lol.