What causes lateral foot pain?

This is a great question, and one with which I have some personal experience. A couple years ago I developed a pain on my lateral left foot, right around the bony protrusion of the 5th metatarsal. It turned out to be peroneal tendonitis. The peroneals run from the lateral knee to the 5th metatarsal. I was doing quite a bit of swimming at the time, and had apparently been pushing off the wall pretty hard on my turns. I was swimming a mile to a mile and a half 3 times per week. It was this repetitive motion that had caused the overuse injury (tendonitis). After some rest, ice, and anti-inflammatory drugs the issue went away. 

This is not the only thing that can cause lateral foot pain. A stress fracture of the metatarsal could also be the culprit. In which case the advice of a medical professional is in order.