I know just what your daughter feels like. I became dizzy a few years ago and it recurs frequently. The best start is with the ENT, and this may help. With me it is a combination of a lot of things, but it turned out to be made worse by allergies, sinus infections (caused by the constant allergies) and migraines. But, if the allergy meds and steroids don't make a difference have her see and neurologists. They can run an MRI to confirm there are no tumors.
I also found with the onset of sudden dizziness that I do suffer from hypoglycemia -- if I don't eat on a regular basis, my blood sugar goes too low and I can pass out. If I have a drop, (which I was doing daily when these episodes started) I will get a migraine, coupled with already my allergies and pressure in my head. So you may want to take her to her pediatrician and have them do a test to see what her blood sugar has been like over the last 90 days. There is a test that can check that and they can tell you if she is having hypoglycemia episodes.
Coupled with all this I became hypersensitive to prepackaged foods, I found that if I had anything with fake sugars I would pass out. Any MSG, I will pass out. So what I recommend is every time she gets dizzy, have her stop and look at what she is doing. Did she just eat something? Is she sensitive to the light, any migraine symptoms? If she is hyper sensitive to something usually her ears will be burning, her cheeks may be red. If so she is having a reaction to something, maybe you can find the source. You can start an elimination diet. See if she can make it happen if she exposes herself to it again.
I hope these things help. If you have any more questions on what next to do, I am sure you can contact me through mamapedia. Best of luck!