Claudication?
http://en.wikipedia.org/wiki/Claudication

I have experienced similar symptoms but in my case the leg/knee actually buckles and recovery by contracting the knee extensors is too slow or not possible; so I fall.
The sensation is like something has suddenly pushed on the knee from the back while I am weight bearing on it. If the knee flexes too much and/or if there is a lot of the body's weight on it at the time, down I go.

In neurogenic claudication there might be a narrowing of the spinal canal due to a disc bulge or spinal oteophytes (calcified extensions) and a particular standing postition or posture temporrily narrows it even further. Standard x-rays or a CT should reveal those defects and help in assessing the cause.

May this be temporary and resolve quickly on its own.

With love, Erika