I Have Small Black Spots Under The Skin Located At

I have small black spots under the skin located at the bottom of my feet. Mainly in the center of my feet. They started out small and have now grown darker and larger. When I they become painful I then notice they have become larger and darker and the pain passes. My physician has no clue what it could be. I also have red and purple dots under my skin from my feet to my knees. What could this be?