I thought I would contribute my story, too, as it sounds similar to your daughter. I've been googling and digging around forums for nearly the last year without anything.
I am an American national residing in Japan. I am currently 27 year old, on the verge of turning 28. One year ago I woke up and could not breathe all the way. One month before that, I experienced these symptoms for about 3 days following a period of intense stress. Before that, I had absolutely no problems with anything. There is simply no better way to put it. I would inhale, and even if I filled my lungs, I would not feel "satisified" with a breath. The breath often feels like it gets stopped at my solar plexus. My condition is constant - from the moment I wake up, to the moment I sleep. I do not have problems sleeping. There is also some sort of colorless phlegm that I cannot cough up, but I have to clear my throat very often. Now in the past year, on top of this breathing condition, I've developed dizziness/lightheadedness when I speak. This gets really bad when I try to walk and talk at the same time - I feel very out of breath. Another curious symptom I would have (that comes and goes), is a pain in my right hand when I do manage to inhale satisfactorily.
The funny thing is, I have no problems as of yet with exercise. I lift weights, can run 20-30 minutes nonstop, exercise regularly (i.e. almost every day) have no problems getting to work and back, etc. The one thing that does KIND of make me feel a tiny bit better, is to lie down, twist my upper torso to the left, and rest on my left elbow. In this position, I would sometimes feel a wheezing in my trachea and feel like air is getting to places it doesn't usually go to. I would then try to cough up the phlegm that I feel like i stuck somewhere, but again it is not a productive cough.
Needless to say, this is a depressingly debilitating state to be in, especially as my job requires me to speak a lot. Externally, I look completely fine, so no one thinks twice. Internally, I may be mildly depressed, though I traditionally have a bright disposition, so that's helping to offset the depression somewhat.
On to the medical part - I've had 3 x-rays, 3 PFTs, a lung CT, 3 ECGs, 2 echograms, 1 24-hr ECG, multiple blood tests, blood oxygenation tests, a laryngoscopy where they had me say "ahhh," presumably to test for vocal cord disorder, through about 4 different doctors now, and nothing at all has been found. I am scheduled for a 24hr blood oxygenation test tomorrow, and some sort of full asthma test next week.
The doctors don't quite know. One said (most recent one), "I've been a doctor for 30 years, and I am sure you don't have anything seriously wrong with you," which is reassuring of course, but doesn't solve the problem of dizziness that is at least a minimal requirement for me to go on with life in some sort of manner, assuming I can live with this breathing problem for the rest of my life. I am currently on Omeprazole, Advair, and very mild hypertension medication. The doctor has given me very light doses of Alprazolam (0.4mg), which helps calm me down.
I've even called the National Jewish Hotline to see if the nurses could give some advice, and they advised me to look into VCD...which I hoped the laryngoscopy cleared up.
I also posted my story in the thread below: