I fell three times last summer. Like you I had no warning when it happened.

It felt as if maybe my knee gave out or that for some reason my legs couldn't support my body (but still they didn't feel weak) or maybe the sidewalk was uneven and my feet couldn't feel the subtle change to compensate for it (?).

And besides the falls, I also had some coordination problems a few times mainly while standing in place (knee wobbling but a neurological feeling/shaking rather than a feeling of weakness).

I went to a Neurologist and found out it's from Lyme disease.