Do hallucinations just begin instantaneously with Dementia?

Mom has moderate dementia and I have been taking care of her for about 8 years but heavily for 4-5 years and living with her in her home. Last year she began having the sundowning agitation around August. In September she was hospitalized for surgery and they tried to give her medication for the agitation and it made her wild and crazy! She was hallucinating, climbing out of bed, they had to use restraints and she chewed them off. She was totally out of her mind for about 40+ hours one day due to medications given to her that did not work. I was told to put her into a NH by her doctor because I would never be able to care for her alone.

I brought her home and got her to a Gerontologist who put her on Remeron and Celexa because Namenda and Seraquel did not work for her. It was a miracle and she was calm and easy to take care of. After about 2 months she developed a headache that continued to get worse and nothing stopped the pain. We tried Tylenol, Advil, Tramadol and they never stopped the pain. We slowly removed her from the Celexa and ran tests to make sure her sodium level was good. She is off the Celexa now but the headache persists.

Five days ago I found out she had pneumonia and she was given antibiotics (Amox TR-K CLV). Today the breathing seemed worse so we took her to the Emergency Room and after 5 hours they said there was NO pneumonia but possibly bronchitis and gave her a Ventolin inhaler.

When we got home we ate and sat down to watch a movie and something just clicked in Mom and she began hallucinating that there had been a child here, then there was a woman, then we had several dogs, then her medication was wrong and she wanted the OLD box of eye drops. It was just crazy stuff, but nothing made sense and none of what she was saying had happened. We told her she had been in the hospital for 5 hours this morning and she called us a liar and told us that she had been home with some woman and we were all gone. The stories kept changing but she was arguing and yelling and telling us that we were liars and needed to get out of her home....we live with her. This went on until we finally refused to speak to her or listen to her ranting.

I have never seen this in her to this degree and never has she thought other people were in the house. It all happened so fast and was so explosive that it was frightening. Each issue just rolled into another issue, one right after the other.

My head was hurting so bad that I thought I was going to have a stroke. My sister has migraines and ulcers and my daughter has had to start therapy to deal with Mom's illness.....today went to an entirely new level.

I am contacting her Gerontologist tomorrow and telling him to get her back on Celexa or something else because I cannot live this way, I just can't!

Has anyone ever seen something like this happen with any family members? Does it happen just this fast....in the blink of an eye? We had sat together all morning at the hospital waiting for the results and worked some "word search puzzles" and we were fine, by nightfall she is telling my sister she has gone through HELL WITH THAT WOMAN ALL DAY LONG! She doesn't even know who that woman was nor does she believe she went to the hospital and refuses to believe she has pneumonia or bronchitis!!!

I know her condition will get worse but this was a huge jump and I do not know what tomorrow with bring. What am I facing here? I thought I had already been in the trenches, this makes me feel like I have been on a cake walk thus far!