Laura, I've had this happen many times before. I don't know if it's a weakness that is causing the muscles to tighten up, which then makes if feel like my left leg will buckle up and I won't be able to move it or if it's something else. My left leg is much weaker than my right. I have more trouble with the left in general (leg, MS hug, arms, and eye). I don't have any suggestions on how to deal with the symptom. I usually just try to shift my weight more to the center or right so I am not putting a lot of weight on the left. It's probably not the right thing to do and an OT or PT may suggest other things but I just seem to do it naturally.