Suddenly can't stop sleeping on my back

I'm 27 weeks and I've never been a back sleeper and have avoided laying on my back for my entire pregnancy. All of the sudden about a week ago, I can't stop sleeping on my back. I fall asleep on my side and wake up on my back, this happens about 10 times a night. I've started putting a pillow behind my back when I fall asleep so that when I roll over in my sleep I will be elevated on one side. Anyone else having this issue? It's so bizarre!