This is a common problem with PCS. It was first clinically demonstrated in the 1970's by Dorothy Gronwall, Ph.D. It is more noticeable if you were to try to do anything that required serious executive processes.

Your muscles can work by demanding more of your lungs but your brain cells struggle to absorb the oxygen because of the lower partial pressure (molecules of oxygen are farther apart). I forget the exact terminology but it is a part of the study required to get a pilot's license.

Even getting acclimated to the altitude by allowing the body to increase the red blood cell concentration does not help the brain like it does the lungs and muscles. Different tissues have different abilities to absorb oxygen. Since only brain cells are not replaced like the other cells of the body regularly are, any damage or toxicity within brain cells can reduce the ability of those cells to effectively release carbon-dioxide and absorb oxygen.

You did the right thing be getting off the mountain. Cerebral edema (brain swelling) can develop from prolonged exposure to altitude, though not usually at only 9000 feet.

I have been sensitive to altitude for decades. I had to give up my pilot's license due to this problem back in 1981.