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There are a lot of reasons why a woman's body might stop producing periods. It would actually be extremely difficult to speculate why they stopped.
Some woman don't show symptoms of pregnancy, although this is rare it does happen.
Producing periods isn't required for survival, therefor when your body is struggling with disease or nutrition it is sometimes one of the things it stops doing in an effort to maintain homeostasis. Things like being malnourished, taking drugs, stress, hormone imbalances, and many more can cause a woman's period to stop.

Her best bet is to go to the doctor, which she's already trying to do. There really isn't anything she can do above and beyond that for now other than trying to be healthy.