It could possibly be a "back dimple" or "Dimple of Venus" (http://en.wikipedia.org/wiki/Dimples_of_Venus).
They're dimples a little off-center from your spine, on your hip, and above your buttocks. I have them and can feel a little lump moving around under there, my doctor told me it's just a little fatty lump called a "lipoma". For a long time I only had one, then recently I noticed the other one had become more prominent and I now have two.
It might be some rotation of the pelvis... Wouldn't hurt to consult with a local chiropractor for examination.
Warm wishes,
AT