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
They tell you to sleep on your side because sleeping on your back decreases blood flow to the baby
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.