While staying with her mother in St. Louis, Maya Angelou was raped by her mother's boyfriend, Mr. Freeman. In chapter 16 of I Know Why The Caged Bird Sings in paragraphs 21, 22, and 23, her thoughts focus on that incident. How does she feel about it? Why did she feel that if Mrs. Cullinan had known about it, she wouldn't have given her "the nice dresses" nor called her a "sweet little thing"?

Let's take a look at the exact passage you're talking about in Maya Angelou's I Know Why the Caged Bird Sings. To set up the scene, our protagonist, Maya, is working in the home of Mrs. Cullinan. From the kitchen, as she's going back and forth to serve Mrs. Cullinan's guests, she hears the ladies talking and laughing:

Whitefolks were so strange. Could they be talking about me? Everybody knew that they stuck together better than the Negroes did. It was possible that Mrs. Cullinan had friends in St. Louis who heard about a girl from Stamps being in court and wrote to tell her. Maybe she knew about Mr. Freeman. My lunch was in my mouth a second time and I went out- side and relieved myself on the bed of four-o’clocks. Miss Glory thought I might be coming down with something and told me to go on home, that Momma would give me some herb tea, and she’d explain to her mistress. I realized how foolish I was being before I reached the pond. Of course Mrs. Cullinan didn’t know. Otherwise she wouldn’t have given me the two nice dresses that Momma cut down, and she certainly wouldn’t have called me a “sweet little thing.” My stomach felt fine, and I didn’t mention any- thing to Momma.

In this scene, Maya is afraid that her employer, Mrs. Cullinan, knows about the fact that Maya was raped by her mother's boyfriend, a fact that Maya would rather keep private. In fact, she seems, at least on the surface, to be more worried about what these people know than she does about the actual fact of being raped. We can guess though that the trauma she has been through manifests itself physically ("my lunch was in my mouth a second time").
Maya gets sick remembering the rape and worrying about whether she'll be socially stigmatized for it. As readers, from this scene alone (earlier in the story, there is much more detail about the sexual abuse and rape that Maya suffers through) we can surmise that Maya feels shame, confusion, and social anxiety in the aftermath of trauma. In this sequence, the social aspect is as important as the private shame.
Maya realizes that her employer couldn't know about the rape, because if she had, Mrs. Cullinan wouldn't have hired Maya. Girls who get raped, in Maya's understanding, are no longer "sweet little things." And they certainly don't get nice dresses to wear. In her understanding of the time and place she lives in, there's a social price to be paid for being involved in a crime of that nature, even though we, as readers, know that she's an innocent victim.