I agree that you should see a doctor as soon as you can... and I would add that if you experience any other symptoms like rash or fever, please dont wait, and go to the hospital.
Did you get bitten by a brown spider? They make the skin dark and leave a crater. Please see a doctor -
Hard to say from that picture. Only things I can think of are bruise or scrape. If the skin's breaking down, then it could be a pressure ulcer forming from shoes too tight over the area.
Your foot and ankle look swollen too. If this isn't going away within a few days to a week, you should ask a doctor about it.