Legs moving uncontrollable then stiffens

My mother in law has recently started doing regular walking and today she fell down. She said that while walking, her legs picked up uncontrollable speed suddenly and she couldnt walk that fast, and then her legs stiffened up and she fell down. This happened on a flat and non-slippery normal surface and she was wearing comfortable shoes. This same thing has happened with her yesterday too and couple of times in last couple of years too. She has diabetes since 8 years but she is on self control and not taking medicines and regular fasting sugar level is 137 and post meal 173. Recently due to blood circulation issue in her feet which was causing burning and numbing sensation in fingers since 2 months, she started eating Gabapin NT 100 and Giloy juice and has been eating since 3 days. Today she went for a walk on empty stomach and had her breakfast after that and we measured her sugar right after which was 179. What could be the reason for legs moving uncontrollable while walking?