Why is Eliza so upset while the colonel and Higgins are talking? (Act 4)

They have been to a garden party, a dinner party, and the opera, and Eliza was extremely successful, fooling everyone. Higgins expresses his contempt for society and says that he is glad that the experiment is over, since he was beginning to grow tired of it. Pickering says that it is almost scary how good at it all Eliza is-she is better than the society ladies. Eliza, who has gone to find Higgins's slippers, begins to look angry, then murderous. She is angry that she has been used in such a way and does not know what to do with herself now that the experiment is over.