Why is it bad to sleep on your back? Even if it's for a little while?

I always seem to wake up on my back so don't worry but I have my pregnancy pillow under me and 3 other so I'm not flat but elevated

Well I hope it wasn't to long I woke up on my back and freaked a little bit I'm usually always on my side

Because the uterus is heavy and can cause lack of circulation , but if you put a pillow on one side you should be fine. Most women still do sleep on their back their whole pregnancy its just a precaution your body will know to move if this happens you will start to feel uncomfortable . you most likely lay on your back without knowing through out the night anyways