Lula is unhappy with Calpurnia because she brought the Finch children to church.
Calpurnia is uncomfortable letting the Finch children go to church by themselves, because she does not think they will behave. She therefore decides to take them to First Purchase church, which is a Negro church. The children have never been there before.
When the children arrive, a woman named Lula is very unhappy to see them. She does not think that white children should be at the black church. She prefers blacks and white to keep to themselves, and have their own lives with no interaction.
Calpurnia is annoyed, and sticks up for the children.
"They's my comp'ny," said Calpurnia. Again I thought her voice strange: she was talking like the rest of them.
"Yeah, an' I reckon you's comp'ny at the Finch house durin' the week." (ch 13)
Lula responds that the white children are not company because Calpurnia is not company when she is at the Finch house. This reinforces the idea that having the white children at their church is inappropriate because they are encroaching on the private time of the blacks.
Another interesting element here is that Calpurnia is code-switching, or using the Negro dialect to talk to Lula. She responds to Lula in the type of language Lula used with her, even though she is perfectly capable to “better” speech. At the Finch house, she talks like they do. At church, she talks like the church members. It is difficult for Calpurnia to fit into two worlds and go between them, and her attempt to bring the Finch children to church with her demonstrates that.