My 91 yr old mother is living w me and she has been having more bad days than good. It's the common sense that baffles me. What is the thinking process....Do they not think things through or have short attention spans? What Do they think about while sitting quietly and unresponsive?
I hear your concern for your mom. I sometimes hear of 100 years olds where there is physical decline but they are still sharp mentally. Please consider having your mom evaluated by a doctor again. I wonder if they need to review her meds? her diet? is she drinking enough fluids? Hard to say sometimes. But I'm glad you noticed the change and are looking for answers to help her.
My dad was getting quiet and eating and drinking less. I thought this was our new normal. But it turned out he had heart failure and he was dying. This is my biggest regret not digging into why and just accepting this was part of old age. It wasn't for him. I hope you can find something to help your mom.
I see that your mother has Parkinson's Disease. Many people with that disease do eventually develop dementia. That, of course, could result in confused thinking and reduced attention span. Symptoms depend on what part of the brain is damaged. It can start in regions that control physical attributes such as hand motions and gait. Later damage can develop in areas that control decision-making, communication, attention, memory, and other parts of thinking.
I suggest that it may be time for Mom to be evaluated by the doctor who is following her Parkinson's, to determine the cause of this distracted behavior.