Sometimes they do...the baby will still be head down when posterior only his face will be towards your stomach....maybe that's not the case.....but the pelvic tilts still help with The back pain :)......
3 weeks ago on ultrasound he was in position, could he have flipped around?
Sounds like baby could be posterior!! Meaning its spine is to yours...try sitting backwards in a chair....pelvic tilts and rocks...anything that keeps your belly hanging....I had horrible back pain because my daughter was posterior and that's what helped me to turn her....keep an eye on the pain..if it gets worse call or doctor or go get checked...
It could be back labor so keep an eye on it and go in to labor and delivery if you think you need to. They will beable to tell you what it is.
And now I'm getting what feels like menstrual cramps, I've been laying on my left side this whole time as women have said it would go away If it wasn't labor, but it's still there, the back pain isn't as bad as it was before, but now I'm getting cramps also.