Why do you think the flower girl was worried when she saw the man with the notebook?

The flower girl is worried because she believes that the man with the notebook is a police informer. 
In the play, the gentleman cannot buy a flower from the flower girl because he does not have enough change. Instead, he gives her what he has, which is three halfpence. Meanwhile, a bystander warns the flower girl that a police informer may be nearby taking notes about her behavior and actions. The bystander tells the flower girl to give the gentleman a flower in exchange for the three halfpence; he implies that she could go to prison for appearing to solicit money from respectable men.
The flower girl becomes hysterical upon hearing the bystander's warning. She demands to know what the man with the notebook has written about her. Meanwhile, the gentleman proclaims that the flower girl has done no harm and that she has certainly not accosted him in any way. He tells the notetaker to mind his own business. For his part, the notetaker denies that he is a police informer. During the exchange, he periodically tells the flower girl to cease her hysterics.
The flower girl becomes worried when she sees the man with the notebook because she fears that he is a police informer. There is also the implication (from the bystander's words) that the flower girl could be seen as soliciting money for sexual favors if she failed to hand over a flower to the gentleman who gave her the three halfpence.