Reason For Having Weak Leg Muscles?

Hi, I have a bit of a medical mystery. My younger sister had an attack about a month ago. She presented with gagging, then had to lie down due to muscle weakness. She couldn t even sit up, she became limp like a rag doll, but still responded to our questions... Although she seemed very confused and didn t know where she was. She was admitted to the hospital due to her state but all of her tests have come back negative. Today she can still not walk without an aid because her balance is off and her leg muscles are extremely weak. She is completely normal when sitting down, but when she tries to walk a short distance she becomes very tired very quickly. She has gotten EEG s, MRI s, blood work multiple times and yet everything still comes back normally. It is seriously affecting her life and we need answers.