Is it ok to lay on my back?

It isn't going to make much of a difference at this point in time. The issue is that the weight of your uterus (when it is much fuller, mind you) can block off two main blood carriers in your body (the aorta and the cava vena). They run down your spinal area directly below the uterus when on your back. This can restrict precious blood supply to your baby. It is not much of an issue when baby is the size of a peapod, but will become more of a thing to worry about around 16+ weeks (this also depends on your progression). You also have some major blood supply on your right side, so laying on your left when you are in your last few weeks is the most ideal (but don't panic about it).

I asked my Doc this same question, and she said, don't worry, your body will tell you... legs will tingle, discomfort to you will happen before issues to your baby.