Hi and welcome.
Please attach a picture of the rash here so that i can give you better advice.
Does this itch?
And how long has this been there for?
Looks like contact dermatitis. Since, this is the surface of the foot that often gets irritated by foot wear, such lesions are rather common and are allergic in etiology. You can start on an oral anti histamine and apply a 1 percent hydrocortisone cream on it twice a day and see if that reliefs this.
Let me know if you have further questions.
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Wish you good health.