Stress Fracture
Bones in the foot (most commonly the metatarsals) can develop barely-visible hairline cracks called stress fractures. These can occur after a long, heavy walk or run to which the person is not accustomed. They heal given time but any underlying problem, such as osteoporosis, poor great toe function or abnormal foot mechanics, should be corrected to prevent recurrence.
On xray the fracture might only become visible when it begins healing.