I'm 37 weeks pregnant and a couple of days ago I sat on the sofa and felt a strnsge twinge in my lower back. It kept coming and going all day. But it only comes when I'm sitting down, but when it comes it feels like an electric shock sort of and goes into my neck and for a few seconds it's like a spasm and I can't really do much except sit still and wait for it to pass..... any ideas??