Waking up with sore back. Help

The past two mornings I have woken up with aches over my entire back. Like I’ve slept wrong, however, I am only mostly sleeping on my back, and not twisted at all.

If I change to sleeping on my stomach the soreness eases after a bit but I’m 18 weeks so I know I can’t do that anymore.

What is going on with me? Also I already know I should be sleeping on my side so that advice is unnecessary. I really just want to know what would be causing this. 😕

Thanks