Why do i keep fainting?

whenever i plat sport or play my clarinet or do any thing that gets me out of breathe, i end up feeling light headed and endup collapsing...when i come round im a little shocked and a tad dizzy but that soon pes and then im fine. it happened when i finished cross country at school and also when i played my clarinet (i have to do a lot of puffing as its a wood wind instrument) and i also ped out when i played badminton. i love playing my sport and im hoping that this is a phase but its been happening for a while now. does any one have any answers?