Carmen - just echoing what everyone else has told you - relax and sleep however you are comfortable. Like cricket said, the recommendation not to sleep on your back is due to the weight of your uterus impeding maternal - not foetal - circulation. So if you were having that problem, you would be uncomfy and want to move anyway.
I was only told to sleep on my left to try and encourage bubs to turn to an ideal position for birth (which, as Kris pointed out - you can do other things to help encourage this). Otherwise it was whatever was comfy and I def. did NOT stay on my left.
Just relax and know that you are doing the best for your baby.