it can become severe if you continue to run on it. it may be something else, but my first concern would be to rule out a stress fracture.
With stress fractures is the pain usually severe? I am able to walk on the foot, and there's no visible swelling.
My biggest concern would be a stress fracture. Stress fractures occurs from overtraining, usually from increases in milage in too qucikly. They can also be caused by poor footwear, runners should change their shoes at a minimum of every three months. See an orthopedist who will examine you and take an xray. If the xray is inconclusive, you will have a bone scan or an MRI. If there is a stress fracture, the treatment will be centered around rest until the bone heals.