Your daughter sounds so much like me when I was in high school. From the time I was 12 until I was 25 I was so terrified to get the flu, or any symptoms of the flu, to the point where I made myself ill.
I would wake up in the morning feeling sick and anxious to go to school, or any public location (I'm having the same issues now, but it's different this time).
A few things which helped me were I cut out all dairy from my diet. It sounds weird, but it helped stave off the nausea. I even avoided pizza because of the cheesy topping. Avoiding dairy really helped with not having any stomach issues. Soy milk was (and still is) a great replacer. Even if she doesn't feel hungry (which is very common), make her have breakfast. I would usually have two slices of whole wheat toast with a very small cup of coffee.
A trick I taught myself whenever I felt sick, was I would close my eyes, and envision the largest brick wall imaginable. And the large brick wall wouldn't be a wall, but it was actually the word "NO". The "NO" enforced in me to stop the thoughts which were causing the nausea, or that actual nausea itself. I mostly envision it in the animation style of "Schoolhouse Rock" google it if you get a chance. ;)
I'm not sure about your religious views... but if the "NO" visualization didn't work, I would then pray "Hail Mary" or the "Our Father" over and over and over and over. Almost like I was crazy. I would say it inside my head though.
Another comforting thought that helped when the praying didn't work, and as an alternative to the "NO" wall, I would tell myself "I am a healthy person, and God doesn't make sick people. Because I am a healthy person I will get through this and I will not be sick." When I remind myself that God loves me and He has a plan for me to prosper and to not be sick, that really helps.
So... tell her to think of the word "NO" or imagine her own mental "stop sign" to stop the feelings of getting sick. Eliminate the dairy from her diet, I can't explain it, but it just works. And if that fails, try praying. Sometimes having a spiritual comforting moment helps more than we know.