Sleeping positions?

🙃

Im currently 17 weeks and I’ve been told it’s best to lay on your side when you sleep. Preferably the left side I guess... and was told that it’s bad to lay on your back? I’ve always been a side sleeper. I could never sleep on my back so I didnt ever think it would become an issue. But why do I feel more comfortable on my back sleeping than I do on my sides?