Hi i also had the same problem in the lower back only in the morning,sometime it will last for 2 hour.And i consulted a doctor and did some check ups,everything was normal.An di have been sleeping on my stomach since childhood,and i never had any pain beforeThen he said check your bed and use Tempur Pedic bed instead of that.I did as he said,and my back pain got over with in weeks.
You just try flip your mattress ,or else go for Tempur Pedic mattress. Hope that would help you.