When wearing high heels, you are forcing your feet into plantar-flexed (pointing down) position, altering the function and position of the ankle joint. This moves your body’s center of mass forward, putting more pressure on your forefoot and your lower body leans forward.
To avoid falling forward, your upper body leans back, increasing the curvature in your lower back and compressing the lumbar vertebrae. This shift in posture changes the normal S-shape of your spine and causes overuse of the lumbar muscles which predisposes you to developing back pain. The more regular the use of high heels, the more the shift is pronounced, and more chronic the muscle overuse becomes. As well, the higher the heel, the bigger the shifts in posture.