Dewey Dell rationalizes that her pregnancy was not her fault. She and Lafe were picking cotton near the woods "and the secret shade," and she said, "will I or wont I [have sex] when the sack was half full because ... if the sack is full when we get to the woods [as they pick down the cotton rows] it wont be me. She means it is inevitable that she has sex with Lafe: "if the sack is full, I cannot help it." Lafe cleverly adds the cotton he picks into her sack, not his, so that her sack is full when they come to the end of the row. Therefore, they have sex, and it's not her fault in Dewey Dell's mind.
Dewey Dell's thought process here reveals her simplicity and gullibility. Her surprise pregnancy is something she doesn't want to take responsibility for, but she doesn't hold Lafe accountable either. It just happened in the way that life will continue to happen to Dewey Dell, who doesn't think she has any control over what she experiences.