Does a stress fracture always appear on a x-ray?

No - it depends on when the picture is taken (before or after swelling has been reduced) and the angle of the picture. If the fracture is small enough and in a spot not covered by the picture, it can easily be missed on a first pass. Swelling with stress fractures can often mask it as well, and it's common for doctors to let the swelling go down some before pictures are taken. X-rays are taken by placing the patient in standard positions for the best viewing angle of the bone being X-rayed; however, that doesn't always mean it's the optimum angle for all injuries.

The only way to know is if the pain and swelling continue past the first week. If the pain and swelling continue past the first week and it's still bruised, they can take another X-ray with contrast dye if needed to help illuminate the area of the potential fracture to help it show up better.