This short sentence has a lot of possible implications for interpretation, and perhaps a good reading of the sentence is a blending of several possibilities. When Ethan arrives home, he knows this is the fateful day of Mattie's leaving the Frome farm, and he is already in a state of distress at the situation. When he comes in he sees Zeena with a book that cost him extra money for postage, so he is a bit more angry than he might of have been in that moment. When he asks Zeena about Mattie, Zeena is completely nonchalant and cold when she replies that Mattie is probably bringing down her trunk. Ethan is then more angry that Zeena isn't even trying to help and that she doesn't seem to be showing any care or concern for what she doing to Mattie by sending her away. This increase in anger could make his face flush. It could also be flushing because he is upset with himself for not being there sooner to help Mattie. He wants to be a gentleman, and he has let Mattie down in that moment. It could also be that he flushed because he feels bad for Mattie and this rise in his emotions towards Mattie is heightened by the rising reality of Mattie's eminent departure. This chapter of the novel is the height of the conflicts within Ethan, and his emotions towards both women are running high for lots of reasons, thus causing his cheeks to flush.