Hi, currently 25 wks pregnant, and have had several times recently when my back has suddenly locked and been really painful, and I've been unable to move. It's only happened when lying down on my back for a moment, or when I've been in the bath. Got totally stuck in the bath and had to yell to get DH to rescue me 
Anyone had similar, or guess what it is? It's very localised to behind my right hip, doesn't seem to be anything to do with my spine. Bit worried about getting stuck!