It may have been because she was the only one in the room who was not invited to the party and she was hurt that Dr. Manfred didn't suggest that she go too or that she accompany him. It looked as though she thought he was lying when he said he had a previous engagment. She didn't have any romantic interest in the Dr. but she expected he would ask her to go and she felt hurt and left out. She may also have been upset that even though he had obviously made his interest for her known, he was obviously quite taken by the beautiful Tracy.
