My legs have been suddenly, inexplicably collapsing out from underneath me...?!

My legs have been suddenly, inexplicably collapsing out from underneath me...?

So, lately I'll have been standing and doing dishes or walking through the mall or what have you, and my legs will have just suddenly, for no apparent reason been collapsing out from underneath me. It usually happens in chunks - an hour or so of this business of me being completely unable to support my own weight. It bears no relation to whether I've eaten or how much sleep I'm getting; my legs are toned and I'm not horrifically obese. Any clues as to what this might be?

1 week ago
I'm going to be at a doctor on the eleventh, so you guys know =)

go see a doctor immediately! This is likely a sign of a more serious condition and shouldn't be taken lightly. Please please do so, I'm serious about this.