Before drastic measures, make sure that nothing has shifted on your bike. At 57, I have had recurring lower back issues since my 20's. I've been cycling since '86 and have noticed similar symptoms as you describe. In my case, it turned out to be that, on certain bikes, my seat tends to cock back a bit too much. I have five bikes, three of them vintage with old Campy seatposts with just one adjusting bolt. These tend to slip when I hit hard bumps going fast, causing the rear of the seat to drop. Even a subtle change affects me. When this happens I adjust the seat back to where it should be and the pain subsides.
Granted, this works for me and clearly not those who have serious disk issues. However, this issue has surfaced on my more modern bikes (Eddy M. Full Campy Record, Pinarello Paris Full Campy R.) with seatposts that are less prone to allow shifting and it took subtle seat angle adjustments to take the "pain" away. Like you, I have a hard time calling it pain...more like localized fatigue.
Hope this helps.