Gosh the human body is complex and still full of mysteries!
Kat for obvious reasons I have been doing a lot of research on CFS (Chronic Fatigue Syndrome) lately and that seems to be a condition fraught with potential multivariant causes. Regardless of any perceived cause, it seems to express with well documented symptoms although the severity can vary. Yet to date any diagnostic testing has not been able to pinpoint a single causative agent or pathological aberration - even though in some individuals causes are found, such as irregular glucose metabolism. Adrenal and thyroid issues are thought to also play a part but very little conclusive evidence has been confirmed to date.
There is absolutely no doubt in my mind that CFS/ME exists. It causes a lot of suffering but is treated by many in the medical profession with disdain. It is a medical enigma and cannot be packaged neatly with a neat cure, so it is more often than not, classified in a derogatory way as being a psychiatric condition.
In a similar way because your daughter does not fit into a neat diagnostic box doesn't mean that there is not something physiological going on. I know you have searched and been a strong advocate for her, yet all roads seem to have lead nowhere.
I know too I haven't answered your question about the nausea and vomiting, although it does sound to be a somewhat extreme reaction to fatigue. I encourage you and her to keep looking for answers but keep enjoying life as best you can in the meantime. One day, either other symptoms will emerge which will make the picture clearer or someone will put their finger on her problem. I pray that this happens sooner rather than later for her.