My case may or may not be similar to you. I'm 41 and started back weight training in January. In March I started getting real sharp pains in my lower back, below the waistline. I had an X-Ray, MRI and started deep tissue therapy. Nothing showed up on any of the films. My massage therapist recommended a chiropractor. He did his exam and noticed that my SI joint was pretty much locked up, which caused inflammation, hence pain. He's been adjusting me 3 times per week and I'm seeing results. It's going to take time. I can already tell a big difference, especially on my right side (where the pain is). I was never as flexible on my right side.
Rehab? Stretching was never a thing I practiced. I do it religiously. He told me the way to prevent it in the future was stretching and maybe doing some core body stuff.
I hope this helps. | 
