Ahhh! The dreaded lunch period! I remember that myself. There were entirely too many people around me and too much noise. I always felt like I was going to start choking on my food. I have a hard time swallowing anything when I am anxious. NOT FUN! I really feel for your daughter, as I was in the same situation. It's very hard, and it gets depressing, that you can't do the things you used to be able to do without a constant feeling of anxiety and fear. You always feel as if you are an outcast, because nobody really understands what you're feeling, especially at that age.
To answer your question: "Do these just come on out of the blue like this?" The answer is YES! You can be sitting there watching a TV show, taking a shower, reading, etc... and they just come on without warning. It is called a Panic Attack, and it's VERY frightening.
Best of luck to both of you with everything, Fox