The psoas portion originates from the frontal aspect of your spine, in behind your stomach and intestines, and is joined by the iliacus portion as it passes the iliac bones of your pelvis. Together, they attach to a bony lump on the inside of your upper femur (called the lesser trochanter). These muscles cross the hip joint, which is where they have their primary function (hip flexion).
The psoas attaches, as I said, to the front aspect of your spine. It arises from the main bodies and transverse processes of several vertebrae in your lumbar spine. When they are in a lengthened position (when your hips are extended straight out), that extra tension is going to add strain on the joints between the vertebrae of the low back, basically pulling them forward and increasing the curve in your lumbar spine. This causes jamming between the facet joints, irregular pressure on the intervertebral discs (IVDs), and changes the way your pelvis and hips orient themselves, leading to muscular compensation, altered biomechanics, and often pain or discomfort. If it happens on one side, it can also lead to spinal torsion, making it easier to injure the ligaments in your lumbar spine.
Another thing that will greatly improve things is (unfortunately, because everyone hates it)strengthening your core and glutes. These muscles counteract the lumbar hyperextension and forward tilt of your pelvis by pulling up on the front of the pubis (the abdominal muscles), and pulling down on the posterior ilia (gluteal muscles). An assessment of your body mechanics by a certified practitioner may reveal weaknesses in the hip and abdominal region. Once these specific muscle weaknesses are diagnosed, a corrective and specific exercise regimen can be provided for your individualized case and needs.