Hi i find this is the case to.
Ive cut and pasted an article i found maybe that explains it.
"There are some people in the medical profession who believe that lying on your right hand side is best avoided while you are pregnant. You have a major vein, called the vena cava, on this side that is responsible for taking blood back to the heart and pressure on this may reduce your blood supply and cause dizziness. If possible position yourself in bed on your left-hand side. "