There are three main hip flexor muscles.  Two originate from the lower anterior spine and attach to the upper inner part of the femur (leg bone)   The other hip flexor is also referred to as a quad muscle.  This muscle (rectus femoris) is located on the anterior part of the thigh and travels from the hip to just below the knee.  

When these muscles become short they cause an exaggerated curve in the low back contributing to low back pain.

To stretch the hip flexors:  Sit on the edge of your bed so that you are balancing on the edge of your gluteals, then take one leg up to your chest and slowly lean back until you are flat on your back.  Focus on dropping the free leg towards the floor.   An alternative stretch is to position yourself on the floor with one leg bent at 90 degrees in front of you.  Ensure that your knee is in line with your ankle and not in front of it.  Extend your other leg behind you until you feel a stretch in the front of the thigh and up into the front of the hip reg