Sorry you are having problems. Could be the way you are sitting, and you may need to make some adjustments. Here is a link that may help (I have no affiliation with this group, just found it surfing).
http://www.ergonomics.com.au/pages/4...how_to_sit.htm
You might however, consider a visit to your GP...sometimes back pains are "referred" pain from another area. For instance gallstones can cause terrible back pain (ask me how I know!). Anyway, hope you feel better soon. Don't forget to hop up and do another activity every so often...sometimes it's easy to get in the zone and stay in the one position too long....guaranteed to make you seize up! Happy quilting.