K.M.
They say you should sleep on your left side because it promotes the best circulation. I always tried to go to bed on my left side but pretty much always ended up on my back. Initially, I freaked out too (I'm a first time mom too) but in the end of your pregnancy, you will just sleep how it's most comfortable. I never had numbness on my back - just sometimes lower back pain and general discomfort, so I would move to my side. Your body will tell you! In the meantime, you can try pillows to prop you. Good luck!