Horrible back pain when sitting?

I'm 32 weeks. Doesn't matter what kind of sitting (good posture, bad posture, lounge posture) or how comfortable the chair is, restaurant booth or couch or stools, I have horrible back pain. Starts midway and goes up, and then down and spreads all around and it's horrible. And then I stand up or lay down and it mostly fades away. This happen to anyone else?