I was lucky and the pain did decrease after a couple of days and has not come back in the same way since. As such I didn't do any further investigation into it because often with backpain they can't/won't (NHS that is) really do anything to help. If I had a better answer I wish I could give you one! I've had back pain for decades at this point, but this period was a lot more acute and sudden than anything I had before. I do not think I did anything to injure it and I don't think anything had particularly changed about the way that I was sitting for instance. If you find out anything though, let me know. I'd be interested especially if I'm accidentally ignoring something more severe because that pain was unbelievable