My lower body skin is much darker than my upper body. Is this a skin disease? Or should I just try some bleaching cream? (photos

The first picture shows the color of my upper body including my face, neck, arms, shoulders, and upper abdomen. Everything under that is way darker, even darker than it is in the 2nd and 3rd photos. Those are pictures of my side and lower back. My feet are also the same color as my face, it looks weird because my calfs are so dark. I went to the dermatologist, spent $175 on a consultation, and all she could say was "I don't know what's wrong." Please help.