What Causes Dizziness While Bending Down?

My daughter is 12 years old and everytime she bends over she gets dizzy. she s and athlete and plays basketball at least 3 times a week. she s also been complaining about chest pains and dizziness periodically and gets tired a to quickly quite often when she s playing basketball. I have taken her to the dr. office about a year ago and she had a full physical and they reported she was actually very healthy only thing they said was she was lacking vitamin D. they did all sorts of lab work and monitored her heart and everything came back negative. Can tell me anything that could possibly be wrong that they may have overlooked or could this possibly be from a lack of vitamin d?