She will soon be 90, frail and doesn't move around much. It was noticeable different than her usual color. She even had on a peach shirt that looks nice on her.
We aren't doctors here (most of us) and not seeing your mom in front of us, it's difficult to know what to tell you. Dehydration, lack of oxygen, low bp all spring to mind. But talking to her doctor or taking her to urgent care will get you an actual answer and not a guess.
I found with my mom in her 90s that she could have good days and bad days (or even weeks). And there was never any real correlation with anything that I could figure out. I wouldn't freak out if your mom isn't showing any other symptoms, just keep a close eye on her to see if anything else starts to happen.
Dehydration, lack of oxygen, low bp all spring to mind. But talking to her doctor or taking her to urgent care will get you an actual answer and not a guess.
Can you do that?