I'm 30 weeks and find myself kind of laying on my back as well. My doctor told me that my body will tell me if I need to change positions and usually it does because I will lose circulation in my arms or legs, so I change positions at that point.
They tell you to sleep on your side because sleeping on your back decreases blood flow to the baby
I slept on my stomach until I was about 20 weeks and I still sleep on my back I have a pillow on one side though so im kinda sideways