My sister fainted at basketball practice?

she is 15 and today at basketball practice, she fainted.... she has felt dizzy at practice before...she never feels that way at games! she has been losing weight for a couple months now... steadily... and is muscular! she isn't eating alot of sugar.. she eats a bowl of cheerios for breakfast and a sandwitch for lunch and a 600 cal dinner. she excercises daily for1 hr 45 min about and after she fainted her heart rate was like 47 beats per min... can anybody help? she is about 5'7" and 130 lbs and going down... thanks!