It's because your uterus is bigger now, so when you lie flat on your back it presses onto the big vessels that run to your heart, so your heart has to work much harder to keep the blood flow going. Will get worse and worse as your tummy grows, and if you do it a lot it can mean that not enough oxygen gets through to your baby (don't worry, I don't think it would have had that effect at this stage yet!). For that reason you should avoid lying flat on your back. Apparently, lying on your left side is the best option, but lying on your right side is also ok, or if you can only sleep on your back than prop yourself up with a few extra pillows.