Stress stripes are not always white, this is what they look like: (they're horizontal)
If they look like this: (vertical)
then that just means she's ready to breed.
I have a super hyperactive female that gets stress stripes even at a mere water change, and then I have another who's never gotten stress stripes in her life. It all depends on the fish's personality.
Does she have a heater? If not, temperature fluctuations can cause major stress and make the fish more susceptible to disease. If you don't have one, I suggest you get one as soon as possible. Bettas prefer a temp of 78-82 Fahrenheit.
If you do have a heater, then it could be something outside that's bugging her. You can try taping white paper around her tank. Other than that, I suggest you simply leave it be.
Also, you can check out this thread: http://www.bettafish.com/showthread.php?t=39134