Why would woman accustomed to wearing high heels experience discomfort when wearing low heels?

Law of Davis: When the soft tissue of the body (ie the achilles tendon) is placed in a certain position for prolonged periods of time the body will accommodate this position. In the case of the achilles tendon, when in a shortened position from shoes with high heels, after time the tendon will become resistant to stretching and cause pain especially where the tendon attaches to the heel bone.

Tendon tightness: High heels alter a person's gait along with changing the stress placed through the foot. Chronic high heel wearing can lead to tightening(shortening) of the calf muscles/tendons over time. Going to a flatter shoe can then lead to these tightened structures becoming irritated or shifting stress to other foot structures causing pain or irritation. Dr l.