Why Does My Elderly Mother’s Walking Ability Fluctuate Drastically From One Day To The Next?

my mother is 93 years old,she has problems walkinkwe have a therapist working her legs and we just figured her to be weak,she was in nursing home for about a month with an infection ,she has back problems ,I guess disintegrating or stenosis ,scoiosis ,unsure,now the strange part,yesterday during therapy they asked her to walk and she walked across the house 4 times with her walker,today she cold barely make it 2 steps,so my thoughts are its not muscular but nerve related,my thoughts were to do an EMG,maybe we cant cure it but at least we will know the cause,nobody seems to have any suggestions but if she can make it to the bathroom that is huge! Any suggestions would be greatly appreciated.