Could be the way you sleep or could be your mattress. Do you work out regularly especially your core muscles? Being out of shape can contribute to that greatly. Could be your age and that your spine is beginning to show signs of aging as well (and this can happen at any age).
Any nuraber of things could be part of the problem. I would suggest making some changes and see what helps you if any.
Then if you still continue to have the pain.. you may want to follow up with a doctor.
