I've always thought it was on your back hun - as far as positioning goes, some do say the left hand side is ideal, but either side really should be fine. I think there are organs on the right that can be a bit squished on the right but having said that I've slept on my right for most of this and my last pregnancy and it was fine, whereas the minute I lie on my back I feel ill!
The Vena Cava is the vein that can be cut off by bubs weight and cause nausea and dizziness - particularly as you get bigger and occurs if you lie on your back. HtH!