What Causes Chest Pain,tiredness And Breathing Problems?

Hello.
I need some help over the health of my daughter, she's going to be fifteen in a few months and seems to have some breathing problems.She specifically complained to getting tired out easly, she goes as far as phanting when she returns from the store, which is roughly ten minutes away from our home.She complained to harsh pain in her chest - as if being stabbed, by her words - and she oftenly says she can't breath.She's even fainted the other day when I was on the bus with her, it took her at least ten to fifteen minutes to wake, after it despite my attempts of waking her.An ambulance came to pick her up, but just as our doctor, they said it was from low blood pressure, but I can't help then to worry.Perhaps it is connected? She has had bad appetite for at least five years now, we can barely get her to eat anything.When she finaly manages rising her weight to normal, it takes her about a month or so to fall 7 kilo down, before getting back onto a proper weight track - which takes alot longer then a mere month for her.She was checked for asthma, and doesn't have it.She's anemic for as long as I can remmeber, and her curent doctor doesn't give us any other information other then the words ''I don't know whats wrong with her''.This causes me alot of consirn, as I am home with her daily.She doesn't have bulimia or anorexia because I would see if she was throwing up, or pushing food away.So far, she's forcing herself to eat, to gain weight, but still claims her chest pain is her biggest problem.The doctor also said he couldn't hear a heart murmur, so it's not likely a heart problem - or thats as much as I understood.