Apparently (according to K) I have an unstable SIJ. So that means another round of Googling (and mental adjustment) to try and deal with the problem. According to http://mcr.coreconcepts.com.sg/the-unstable-sij/, a sacroiliac belt might help :(

This PDF is pretty authoritative too: http://www.capitalregionneurosurgery.com/cm.pdf.