Start keeping a daily journal (I know, I've said this before. I think journals of symptoms can be such an eye-opener). Get a legal pad and take notes. When your daughter has a symptom, write it down, and write down what's going on around her at the time. For example: 9 am. Vomiting and headache. She just ate ____________ (note the food, drink, and time) and is in _________ (note the room). Describe what is happening around her in brief words.
You may notice a pattern, but even if you don't, it will be helpful for a doctor.
Some questions: what kind of allergy testing did she have? Has she had the Celiac testing (blood panel and biopsy)?
Does the blacking out happen when she stands up, after lying down or sititng? My daughter has a condition called Postural Orthostatic Tachycardia Syndrome, where she gets dizzy and can pass out because her heart rate goes extremely high when she stands up. She also has Gastroparesis with similar symptoms as your daughter.
Have you consulted a Gastroenterologist (GI specialist)?
Don't stop searching! You are not alone.