Kathe,
Most schools have athletic trainers and team doctors that should be able to treat you or refer you to a professional. Listen to them and do what they say.

go see your trainer at school, they are always a big helpp !

As the previous answers hinted, this is not a good thing. Sounds like it could be structural (muscles, ligaments or tendons), or nerves, or blood flow, or something I can't even imagine. Definitely get some professional attention, they may be able to identify the source or circumstances causing the collapse and/or recommend adjustments in your training. Good Luck!