This condition can occur when there is a fracture to the bone that cannot be detected initially by an x-ray, but shows up later with healing evident.
In this xray, an elderly female presented with no known trauma but pain in her heel. She had tenderness when the heel was squeezed from side to side. Given her history of osteoporosis, a stress fracture was suspected, but was difficult to confirm on x-ray.
One month later, however, the healing could be seen on a repeat x-ray, looking like a cloud or cotton ball forming in the middle of the bone. At this visit, she was also feeling better.
Here is another case where, in the x-ray on the right, the fracture is not visible, but the later film shows healing as indicated by the white arrow.