pain that is present only at night when you are relatively inactive tends to be non-mechanical in nature. It is not a simple back ache and should be followed up with your doctor. It may of course be related to an activity that you do during the day or perhaps your mattress, but to be sure I would contact your doctor :) good luck!
It sounds like the way my back problems started. Docs said it was arthritis after xrays and MRI. I saw a chiropractor and that helped for a while. Now I'm considering pain management.
I'm all too familiar with this pain. I used to only get it there when I was asleep, too. Chiropractic has been the only form of relief for me that works.