I am of the belief that a lot of mental health issues come from complex trauma. Not PTSD, but complex trauma. This can be emotional, physical, psychological or any kind of abuse that has layered throughout the years. I do not know the history of your daughter, but I have been working with a trauma therapist for over 6 years now. My body is stuck in fight/flight and I get activated even when there is no obvious "threat." If physical illness has been ruled out, maybe ask if she is willing to see a therapist, specifically one that is trauma informed. Does she like to read? Do you? There is a book called, "The Body Keeps the Score," written by Bessel van der Kolk. I recommend this book. Just a thought. I hope the best for both of you. I understand severe anxiety and can empathize.