Has my mother's cancer gone to her brain?

I am the primary caregiver to my 89 y/o mother. She has stage 3b lung cancer. She also had 10 palliative radiation treatments that ended Feb. 1st. Her condition is about the same if not a small amount of decline physically. However, mentally she is declining. She gets confused easily, misspells words on grocery lists. Like spinnach, and 400% milk and 1/2 and 1 1/2 milk. She gets very anxious over small things. Today, while my brother and SIL are there she calls me very agitated that the nurse's aid hadn't gotten there early enough for her. She was all worried that the nurse was going to come while she was eating breakfast. My SIL and niece both can see a difference in her cognitive processes. FYI, she is on hospice. Is this normal for someone with lung cancer or could it have gone to her brain? Thanks