I can't say for sure what's causing your daughter's sudden symptoms. I developed allergies when I was in my mid-twenties. Prior to that, I had no problems with allergies, none when I was growing up. My mom had allergies all while she was growing up, but they went away when she was in her thirties. Allergies are weird like that.
I know my post wasn't particularly enlightening, but my point is that allergies can develop at any time. I don't know where you live, but the pollen levels where I live have been pretty high for the past 3 months. Pollen.com is a good website to check out pollen levels. You can enter a zip code and it'll give you a "5-day pollen forecast" and will tell you about the predominant pollens in the area. They also have an allergy symptom tracking tool which may be helpful if you're trying to determine whether you have allergies. You can easily compare your symptoms to the pollen levels.
Hi. I am 46 and never had any respiratory issues until 2004, when I was diagnosed with acute asthma like your daughter--out of the blue. I take the same meds she does and also use a nebulizer (breathing machine) 4 times a day along with 2 COPD meds even though I don't have copd. I am also allergic to 13 trees, grasses & weeds and take zyrtec or claritin daily. Attacks are pretty scarey, as the tightness doesn't allow you to fill your lungs even though you're physically inhaling. Personally, I ask the doctor for a peak flow meter, which is a plastic tube designed to measure your flow intake/output. It helped my doc when I Googled & download a free asthma log/diary & recorded my flow 3 times a day. Aside from documenting patterns from environmental triggers, it showed how bad my flow gets during attacks. It also helped design an asthma action plan that has been spot on ever since (well contolled with meds--no more er visits. I went from being my respiratory doc's "most hard core asthmatic" with monthly appointments, to just a regular patient seen twice a year. Hope my story helps.:-)