Your hip flexors are made up of many different muscles including the psoas, iliacus, rectus femoris, sartorius, tensor fasciae latae, adductors, and gracillis. The major hip flexor muscle is two muscles combined called the iliopsoas. These muscles work to move your upper thigh (femur) to your chest. When this muscle is injured, it can cause pain even with walking. Here are some stretches for your hip flexors.
First start in a kneeling position. Place the knee of the hip flexor you want to stretch on the ground, and take the opposite leg and bend it in front of you at a ninety-degree angle. You might want to place a pillow under your knee to protect it. You will be in a lunge position with the knee of your injured side on the ground. Now drive the knee of the uninjured side forward away from your body. Hold for 30 seconds, and do this 3 times.
Next, you will be on your stomach, or in prone. Bring your foot of the injured side towards your buttocks and grab your ankle. Gently pull your foot towards your buttocks and then lift your thigh off the ground pushing your leg up in the air. To help lift your hip, push your foot away into your hand. Hold for 30 seconds and do it 3 times.
Finally, lie down on your bed or a high surface. You will need good balance for this so you won't fall off the bed. Slide to the side of the bed with the injured side hanging off the edge. Now bend the opposite knee towards your chest and hold onto it. Just relax your leg off the side and you should feel a stretch. Hold for 30 seconds and do it 3 times.