This sounds like a little overuse in the tendons connecting the muscles of the back and the pelvis.
As it is one-sided, I wonder if you have tended to carry and load one side in preference to the other over the years?
One of the reasons pain is eased upon movement is better circulation and production of pain killing chemicals and you find it is aggravated by periods of sitting or certain postures.
Hopefully it has a simple cause and I'd try some Arnica Gel for a few days initially.
If it persists after that, use massage and magnesium to help relax and release those tight muscles.