In Mrs. Whaley's debut, she introduces her class to Sustained Silent Reading, which she nicknames Drop Everything and Read (DEAR). Later on in the book, after Ramona gets egg in her hair, she overhears Mrs. Whaley mention that her "little showoff" got egg in her hair and exclaim, "What a nuisance!". This causes Ramona to fear that Mrs. Whaley finds her annoying.
The fear escalates when Ramona falls ill and throws up in class, then afterwards is told that she ought to have "told Mrs. Whaley you didn't feel good". Mrs. Whaley sends her a "get well" card that is signed with a whale and a "Y".
As it turned out, Mrs. Whaley wasn't calling Ramona a nuisance but rather saying that it was a nuisance to clean the egg out of her hair.