Aibileen says: ""But it weren’t too long before I seen something in me had changed. A bitter seed was planted inside a me"" (p. 3). What was the cause of her bitterness?

The cause of Aibeleen's bitterness was to recognize that "seed" for saying so or that new trend or habit that perhaps she has acquired, the good thing was that in a short time she recognized it.

Explanation:

The Aibileen precisely recognized that change that took place within him, which apparently was not for the better, because as he says it was a cause of bitterness, or it could be, that the change involved left some habit or good habit that perhaps it had.

you didn't show the passage, so unfortunately i cant you.