J'ai mal au dos

So Friday morning recess, I was sitting in the staff common room having a nice chat, got up to go to the loo before going back to classes (usual routine here!), still nothing untoward. Close the cubicle door and suddenly excruciating backache. You know the sort where you just can't move?! Well, muscular I think, couldn't even reach the basin to wash my hands without pain. I've had back ache like this before, but normally provoked by something - over exertion at the gym or rugby. But this came from nowhere, so I'm assuming it's pregnancy related. That was 2 days ago. Still in pain. Moving very slowly and in straigh lines only.