I found the exact same thing so I asked my Obstetrician. He said if you're not feeling dizzy & unwell then it's not a problem. But once you hit 26weeks you should avoid being flat on your back all together & make sure you roll to your side once you wake up. He said your body will react if it's doing you harm & you'll naturally roll out of that position, though once you wake up & realise you're on your back, turn on your side (after 26weeks). I now sleep on a incline so that when I wake up on my back I don't need to roll over. I have only been doing this for a few nights, but I'm sleeping much better since I put the extra pillows under me. I have also found that my sinus is a bit clearer from doing this too. Though occaisionally I wake up with a little bit of a sore neck.