You may have an accessory navicular, which is an extra bone
growth on the tarsal navicular, the bone in the arch of your
foot. This can be accompanied by an abnormal attachment of the posterior tibial tendon (PTT) and can cause the sort of pain you are describing. Occasionally, the accessory navicular is injured and separated from the "true" navicular. If this does not heal correctly, it can also be a chronic source of instep pain.