It could be stress. Is she with other fish or has she been recently moved? Is she a new fish? I've gotten lots of comets from stores with stress streaks and with proper care they recover in a few weeks.
Another possibility is ph, a ph crash could kill a goldie and will definately produce stress streaks.
Septicemia would be my final guess, but I wouldn't expect that unless the fish is new or water conditions are really bad.
The best cure for all of these is time, proper care, and water quality.
Anybody who's seen the personality of fish has had their minds and hearts opened to a whole new world.