My daughter (14 years old) just fainted last month while we were on vacation. It was very scary! We were on vacation in the Grand Canyon, and went to Sunday mass. I heard a gasp and then felt something hit me - it was her, falling to the ground. She was out cold for about 5 seconds (which felt like forever). There was an EMT at mass, and he came over. He said it was probably the altitude and maybe dehydration. She was fine after, just bruised. When we got back home I took her to the doctor, and he said it's actually very common for young women to faint, especially when they stand up suddenly, and that dehydration also plays some part in it.
Good luck! Be careful!