A stress fracture may or may not show up on X-ray.  If there is clinical evidence of a stress fracture but no immediate radiographic evidence, then sometimes it takes about 2 weeks for the stress fracture to be evident on the X-ray, as you will see the evidence of new bone formation.  Other times, when there is no evidence of bone reaction after several weeks but a stress fracture is involved, then the patient may have advanced testing such as an MRI to confirm the diagnosis.