That's almost certainly a bad shutter. The shutter may need adjustment or replacement. For speeds above your X-sync (the speed where the shutter is at least temporarily totally open), the shutter curtains are launched with a slight delay to have a slit cross the path of the film. If the timing is wrong, you can end up with the two curtains being sent at inappropriate times relative to one another, and get effects like you describe.
I assume you weren't shooting with electronic flash. If you were, then you need to reduce your shutter speed to the X-sync speed (probably 1/30 sec. on your camera).
Note, too, that 120 film isn't 120mm. 120 is just a format describer made by Kodak (35mm film is 135 under this system) and the system was adopted by other manufacturers. 120 film is actually about 65mm wide.
Enjoy, and good luck!