Is it common for someone with dementia to display two different personalities (with and without a UTI)?

My mother-in-law, Barbara, is 87yo with atypical dementia, heart issues, deafness, and a tendency to get UTIs. Her usual personality is unusually sweet, appreciative, and kindly, but when she gets a UTI, I see a different Barbara. "Barbara B" is just as good-natured, but she thinks she's about 20-30 years younger, that she's living in a different state, and working for Bell & Howell making microfiches. Barbara B. is astounded to hear her age and that she's been living with us for four years. She is more anxious and fearful and her delusion of making microfiches is kind of like she has died and gone to hell. People keep coming into her room and telling her to watch this machine or do that task. She's anxious and harried and unable to sleep. At one point I told her the person she had just talked to was a dream. She said, "More like a nightmare." I should note that while Barbara's memory and ability to form new memories are poor (and her coordination is severely impaired), she is still a very intelligent, alert person. At one point, when I explained that no, she wasn't working for Bell & Howell, she said, "What? You mean I'm not getting *paid* for this!" Thankfully, she never entirely loses her sense of humor.