In 1984, why is Winston scared of the girl from the fiction department?

Winston initially fears Julia, who is a bold-faced young woman that works in the Fiction Department. Winston completely detests her orthodox demeanor and believes that she is dangerous. Being a secret dissident, Winston is cautious around Julia because he thinks that she works for the Thought Police. He believes that Julia is a spy, and he is filled with terror each time she glances at him.
In chapter 8, Winston wanders through the prole section of town and stops in Mr. Charrington's antique shop to look around. As Winston is walking home, the mysterious girl from the Fiction Department walks past and looks directly at him. Winston is frozen with fear and even contemplates brutally attacking her. Winston's anger towards Julia stems from his suppressed sexual desires and fear that she is spying on him. Eventually, the girl from the Fiction Department slips Winston a note telling him she loves him. The two characters meet up and develop a close relationship. Winston grows to love Julia, and the couple carries on their secret affair until they are arrested by the Thought Police in the apartment above Mr. Charrington's shop.

The girl from the Fiction Department, or Ficdep, turns out to be Julia, who will become Winston's lover in due course. However, when we're first introduced to her we don't know who she is. She's simply a very pretty, dark-haired girl who causes Winston "to feel a peculiar uneasiness which had fear mixed up in it as well as hostility, whenever she was anywhere near him."
Winston lives in a totalitarian society in which people naturally distrust one another, always believing at any moment that they'll be turned into the authorities for some crime or other, whether real or imagined. The Thought Police are the secret police force of the Party. They crack down on the slightest hint of dissent or independent thought. Winston is initially distrustful of Julia because he thinks she's a member of the Thought Police. On the face of it, she seems like a fanatically loyal Party member, so Winston gives her a wide berth.
Also, Winston undoubtedly has misogynist attitudes, dinned into his brain by a society which doesn't tolerate expressions of love or affection. He resents the dark-haired girl from Ficdep because she is pretty and sexually attractive and stirs up emotions he thought he'd managed to suppress. How dare she make him feel this way!