when i was the same age as your daughter i went through a stage of fainting. it started when i was about 17 and went through till was 19. i would faint for no apparant reason. it was a bit weird for me and it upset my family particullaly my dad who got really worried especially when i fainted in front of him, i think they thought there was something majorly wrong with me. so i got hauled off to the doctors who did blood tests and all sorts, they could find absolutely nothing wrong with me.
i wasnt so worried because i didnt feel like there was anything wrong with me, and after i came round after fainting i felt fine. i started to get a bit paranoid because everyone around me was making a big deal about it and that kind of made me feel like this was something worse than it was.
after about two years the fainting gradually phased out and i havnt fainted in ages now.
if the tests have all come back clear then i really wouldnt worry, it was a weird stage in my life but im fine now. if your daughter is not overly worried then i dont think you should be either.
hope things went ok with the doctor.