It is minor inflammation of the facet joints, in my case in the lumbar spine. On a practical level, I would describe it as a very sharp local pain on the right lumbar side, very painful when extending either leg (particularly the right), no ability to handle loads in the arms, and difficulty going from sitting to standing and vice-versa.
Four days later it is feeling a little better and more mobile. Sitting down doesn’t help at all, walking around is difficult for long periods: going to have to grin and bare this one. Hopefully it’ll subside soon and the blog will be back in full-force.