Hello, I am feeling for several month a mild pain in the instep but the two last weeks I noticed a rash on this area. What could it be?

Shoes.. It is difficult to say without more information, but is it possible that the insole of your shoe(s) are irritating your arch area, causing both the pain and the rash?
Get checked. Hard to know without examining. Pain may be associated with the rash so the cause has to be determined. It may be fugal, bacterial or yeast. Culture and/or biopsy in office can be beneficial. Get it checked because it may also be a structural issue. Broken bones can sometimes form fracture blisters. See a podiatrist.