What does it mean when you have black spots on the bottom of your feet?

I have been looking for an answer to this question for SOME time. I got mine about 7 years ago..they keep changing shapes/and how many I have...not I am noticiing that they are on my palms.

Been to more than 5 dermatologists. they cant figure it out. Everyone keeps tellin me I have syphllis. I do not. I have been tested EVERY time..and it is not syphillis. at first when it was just the bottom of my feet, I wasn't that upset..although it is very embarrasing!! Now my palms?

I am mortified. cuz people just assume that you are dirty.

--- Arsenic poisoning or pollution is pretty rare ... has that been considered? Or parasites, but the dermotologist should have spotted that.