Cramps are caused by the build up of lactic acid in your muscles. Lactic acid is caused by a lack of oxygen going to your muscles. SO if you get cramps in your foot, that could mean that there is a lot of lactic acid build up in the muscles of your foot. This is not a problem! All you need to do, is make your feet muscles more oxygen efficient!
Try to bend and relax them everyonce and a while, to renew the blood flow and therefore, delivering new oxygen rich blood cells to the area, washing away the lactic acid build-up!