What Does Sudden Weakness In The Hands And Legs Indicate?

My 86 yo mother had sudden one sided severe weakness in leg and hand. Once ambulating without any devised to a wheelchair, learning to walk with walker. MRI s of brain, neck, lower back are all clear. bilateral lower extremity ultra sound is negative, chest X-ray is negative. I am thinking she had a stroke, but there is no indication of that. It has been 10 days, and she is getting physical therapy rehab. Please let me know what else could be happening. Thank you