What Causes Dizziness Along With Headache And Nausea?

My daughter is 21. 18 months ago, after returning from a year in England and Europe, she started getting dizzy spells, usually accompanied by a headache, and nausea. The headache and nausea are usually not too bad but on a couple of occasions have been. sometimes she just gets nausea but no vomiting. she had one bad episode where she cant remember 10 minutes and was in a daze walking around. She is getting milder episodes now a couple of times a week and often has to lie down and sleep it off which is interfering with uni. She has had a MRI of her head and all clear. What course of investigation should we do next? She has auburn hair and fair skin so that sometimes means she is more sensitive. She eats an extrermely healthy diet of little fats and sugar and lots of protein and vegetables.