I am sure that something happened, but doubt you misplaced your sacroiliac. You did check the couch cushions didn't you? The SI joint has extremely limited mobility unless the muscles and ligaments which surround it are damaged. This would almost certainly result in significant pain not just a leaning posture. I offer this
Sacroiliac joint dysfunction - Wikipedia, the free encyclopedia for your reading pleasure. Also this [ame=http://www.youtube.com/watch?v=1iwmcCw4bAw]Sacroiliac Joint Dysfunction Animation - Everything You Need To Know - Dr. Nabil Ebraheim, M.D. - YouTube[/ame] is a good video giving nice visual information and diagnostic and therapeutic options.