Could you have stepped on it wrong and not really realized it? Try to think if you had put any abnormal pressure on the injured area. You very well could have gotten a stress fracture. It is basically just a very small hairline crack that develops in the bone. This is actualy what a “shin splint” is, and you can get them pretty much anywhere. Seriously man, Ice the hell out of your foot. 1. it will reduce and internal inflammation that may and probably has occurred, and 2. it will help reduce the pain. Most inflammation from an injury is caused by an increase in blood flow to the injury site area, and by applying ice you shrink the capillaries which in turn reduces the amount of blood flow to the injury site, which will relieve excess pressure and reduce pain caused by inflammation. There are many very small parts in your feet and you can easily damage them without realizing it, that is why it hurts like hell, it is your body’s way of telling you something is wrong. Go to your doctor and see what they say. It sounds to me like it could be either a pinched nerve, or some sort of stress injury to the bone/ligaments/tendons. It could also be some sort of tendonitis (sp) or similar ailment.
Hope that helps.