Your foot is hurting during running but not so bad that you are terribly worried. You ice it as you know you should for any acute injury. The next day when you walk the foot is a little sore, but not so bad. When you try and exercise the foot does hurt more and you cannot run on it. You should be concerned of a stress fracture.
What is a stress fracture? A traumatic fracture occurs from a serious injury such as kicking something or a heavy object falling on your foot like a dumbbell or a bowling ball. A stress fracture occurs from repetitive micromotion of the bone. Similar to repeated bending a paperclip, the bone can eventually break from the micromotion of running. The muscles that support the foot are fatigued and can no longer protect the bone adequately. It can occur from running with worn down shoes or on too hard a surface.
Females are more likely to get stress fractures than males. Stress fractures can occur at any age and I have seen it in children as well as adults. It may be only mildly painful but is typically worse with activities.
Stress fracture can often be diagnosed on x-ray. However, because this is a hair-line fracture it may not be visible on the x-ray. An MRI is the most sensitive test and will show any stress fracture if there is suspicion but nothing seen on x-ray.
A stress fracture is treated by resting that bone. This may require a protective boot or a shoe insert. It will typically take several weeks to heal. If you return to activities too soon, it will refracture. Healing is best checked on examination and follow-up xrays and sometimes a CT scan. A bone stimulator can help accelerate the healing.
As with most other injuries it is important to ignore ongoing pain. If pain lasts more than a few days and it is not improving, see your local orthopedic sports specialist and get the right diagnosis.