This is tricky to explain, so here goes....
When I sit down, particularly on more formal style straight back chairs (office and dining chairs), if I am sitting for anything more than a short while, when I go to get up, oh my word! Pain! It hurts when I get up, as my bum is being lifted off the seat if that makes sense, a weird pain at the very very bottom of my spine, so feels like the top of my bum, slightly off centre to the right (love a bit of detail) So if I sit at an angle or put my leg under my bum and sit on that it helps loads.
That sounds weird I know, has anyone else got this?? I think I vaguely remember it with my first but this is more regular and has been going since earlier in the pregnancy.