Yup its fine. I was still on my back at around 36 weeks - until I felt a bit dizzy or like I was going to puke...

Your body knows what is good for it - so just move if you feel strange. Sometimes I could lie there for ages before feeling it.. sometimes I felt nothing at all.

Once it even woke me up cos I had rolled over and I thought I was going to puke - so I just rolled over onto my side and I was fine.