Is mom mean, or affected by cancer & Chemotherapy/?

Mother is 68 years old divorced, and has had cancer cancer, which spread to stomach area for 10 years. She has been on Chemotherapy. I took her, my girlfriend and her mother to a restaurant. We were talking and I quoted something a professor had said in college, my mother said " YOUR MARKS WERE NO GOOD" No one said anything. The next day I asked why she said that, and she denied saying it, and yelled at me. Can her saying such a strange thing be caused by cancer, or Chemo? Also she does not know what my marks were.