What causes dark spots at the bottom of feet?

Skin color changes: You would want to look at the pigmentation and ask yourself. How long has it been there, has it grown over the past week/months, has it changed in size/color, etc. There are many different reasons to have pigmentation on your skin. You want to be weary of any new or changing skin lesions, especially if you have a hx of skin cancer. See your pcp/dermatologist for follow-up care.

Maybe you stepped: On something without realizing it. But melanoma, a deadly pigmented skin cancer, can occur on soles, too. You should have a doctor look at it.