Joint pain

my child is only 14 but suffers form joint pain. her knees crack and crunch every time she moves. Lately she describes them as being jammed unable to crack - they click but cant crack and it hurts her. it often feels bruised and jarred and finds it painful to walk. these symtoms come and go . Also her hip cracks and crushes when she moves it and when she over exercises she can be in real excrusiating pain. Doees anyone no what i meen. she has had xrays and a mri on her hip but doctors cant find anything wrong HELP