Why was Jane Fairfax so sad during Mr. Knightley's party?

Thanks for the answer! I did watch the whole movie, yes, I loved it. I felt like with this movie, so much was expressed simply through small glances and mannerisms, I wasn't always sure what was going on. Maybe those who have read the book understand the whole thing better. Another scene that left me with questions, was the one where Emma plays the piano, and then Jane follows her and obviously outdoes her. While Emma plays, the camera is focused on Jane a lot, and she has these different facial expressions, maybe worry, maybe relief? The whole thing is so subtle, I am not sure. I am certain the movie director meant to express something for us in that scene, I just don't have any idea what it could be. And 10 days after seeing the movie, I am still wondering about it.