Why is the flower girl so afraid of the note taker? (Act 1)

Eliza is afraid he is looking to get her into trouble with the police.

THE FLOWER GIRL [springing up terrified] I ain't done nothing wrong by speaking to the gentleman. I've a right to sell flowers if I keep off the kerb. [Hysterically] I'm a respectable girl: so help me, I never spoke to him except to ask him to buy a flower off me. [General hubbub, mostly sympathetic to the flower girl, but deprecating her excessive sensibility. Cries of Don't start hollerin. Who's hurting you? Nobody's going to touch you. What's the good of fussing? Steady on. Easy, easy, etc., come from the elderly staid spectators, who pat her comfortingly. Less patient ones bid her shut her head, or ask her roughly what is wrong with her. A remoter group, not knowing what the matter is, crowd in and increase the noise with question and answer: What's the row? What she do? Where is he? A tec taking her down. What! him? Yes: him over there: Took money off the gentleman, etc. The flower girl, distraught and mobbed, breaks through them to the gentleman, crying mildly] Oh, sir, don't let him charge me. You dunno what it means to me. They'll take away my character and drive me on the streets for speaking to gentlemen. They—