What is this on my foot?

I had something that looked like that on my hands for about a week. Maybe you're allergic to something. If I were you, I would try buying a new pair of shoes to see if it goes away (unless you already did that, which you probably did). I doubt that this could be it, but maybe some of the soap you use to clean your clothes is staying in your socks or something, so you might try using a different kind of clothing detergent stuff.