Hi, My Daughter Is 16 And Has Been Having Trouble

Hi, My daughter is 16 and has been having trouble breathing. She feels like she can't breathe like she's not getting enough air to her lungs. This has been going on for about a week now. Her other symptoms are dizzy, light headed, feeling like she may pass out and sometimes she has headaches. I have taken her to the Dr., she has had chest x-rays which came back that her lungs were clear, she's had an EKG that came back normal. She has also had her hemoglobin checked and that was normal. They aren't sure what is wrong with her Also, I'm going to add this not that she doesn't drink a lot of sodas and she does however drink plenty of water, and she eats normal and hasn't tried anything new and as far as we know she isn't allergic to anything.. I guess my question is what your opinion may be on what we should do next?