Please tell me what can cause pain just over the instep in the foot?

Pressure from a shoe: If you are referring to the top of your foot, you may have a high arch or a bone spur from one of the joints that rub against the top of the shoe. You may need an x-ray, try "deeper " shoes. Could also be a contusion or bruise.

Instep: There are several possibilities here . Tendonitis, bone spurring due to jamming due to cavus foot type . An evaluation and x rays by your podiatrist will answer the question.