Progression to stage 4 within five years

Hello. I have been helping with the care support for my fiance's mom. She was diagnosed with Parkinson's five years ago. I have really looked for answers regarding her condition- mostly why? and how? Since being diagnosed she is now reduced to being practically bedbound, although she is allowed to walk with assistance, she denies that there is anything wrong and will try to walk alone and do daily chores that she just can't do any longer. She can hardly speak often. I am not seeing that this is the regular rate of progression.