weak knees?

for the past year, that I remember, dd has been having "issues" with her knees. We'd be walking and one, mostly rt, would "give out". I'm not sure how long this has been going on as I probably though dd was just clumsy. Now dd is becoming very vocal about how painful this episodes are though the pain goes away within a minute. Just this am dd woke complaining about her knee issue which really is odd as she was asleep in bed. After a quick rub she went back to sleep.
anyone BTDT?