I wonder if any of these trigger points might be the cause?

http://www.pressurepointer.com/Serra...ger_points.htm
http://www.pressurepointer.com/Latis...ger_points.htm

the main chart in case I missed another possibility -
http://www.pressurepointer.com/pain_reference_chart.htm

If you can press a tennis ball against a wall or lay on the floor and roll on it or any small ball -
or have a friend help - press around on your back in the areas shown to see if any small knots or specific sore spots are there.

If you find any - here is the user guide on how to resolve them-
http://www.pressurepointer.com/Users1.htm