The first thing I would recommend is having the knee checked. Sensory problems, balance issues, weakness, spasms... Things that may have you putting a wrong step in one too many times and/or plain old wear and tear can lead to tears in the connective tissues that support the knee. Also ligaments and tendons don't have nerves so they can be torn without pain.

I have torn the ACLs and MCLs in both my knees and they do feel as though they might buckle (and well they could) frequently, but not all the time. It does come and go with stretches of time where I almost wonder if the ligaments regrew. Times where I wonder if they'll ever be able to support me again, and everything in between.

Ironically it's the left leg for me as well that seems the most inclined to buckle if I shift myself too far left. The right leg is a bit more amiable about the abuse, but it also generally doesn't give a warning feeling of about to buckle when it's had enough, it just collapses leaving me stuck on the ground till it decides it's ready to play again.