The kneeling iliopsoas stretch is a very helpful stretch for low back, hip and pelvis pain. This muscle attaches to all lumbar (low back) vertebrae, so this is very important for low back and pelvis related pain.
Be sure to always kneel on something soft, like a yoga mat, and hold onto something stabilizing. Kneel on the affected side, stabilizing with the opposite hand and leg. Lean in and twist toward the opposite knee while keeping your back arched. At this point you should feel a great stretch through that iliopsoas muscle.
Be sure to hold long enough for a good stretch (at least 10-20 seconds... more if it's accessible for you). Repeat throughout the day, morning to night, as often as your schedule allows while your body is hurting.