I Am 63 Male. All Across The Bottom Of My

I am 63 male. All across the bottom of my lower back is extremely painful. Once I start walking I am ok, but if I sit or lay for any period of time, I can hardly stand and walk. I have had strained muscles in my back before but they always went away with rest in a few days. This has been going on for over 3 weeks and is not getting better. Your thoughts