When we think of stretching, we think of BIG stretches that go so far they almost hurt. There's nothing wrong with a big stretch, but there are very few big stretches that will get to your lower back muscles, and little stretches or reminders of forgotten movement can do even more good.
Here's a Feldenkrais way to "stretch" or free up your lower back:
Lie on the floor/yoga mat on your back with your knees bent comfortably. Cross your left leg over your right leg at the knee. Very gently, begin to lean both knees to the right, just until your left hip comes off the floor. Stay there for a moment, then come slowly back to neutral. Go back and forth between leaning a little and neutral a few times, breathing easily the whole time. Repeat on the other side, right leg crossed over left, leaning to the left.
For a slightly larger movement, cross your left leg over your right and place your left hand on your left thigh. This time, as you slowly lean your legs, let your hand, arm, ribcage, and head follow. Go back and forth between leaning and neutral a few times, still breathing. Repeat on the other side.
Be patient with yourself. Go only as far as you want, not as far as you can or "should". There is no "should" in the world of Feldenkrais.