I too hope you get somewhere tomorrow. I also hope the downturn is caused by a UTI and that once treated your mum returns to her previous self.
You do need more help. Maybe some respite care would be a good idea, this will give you some breathing space , Adult Social services may be able to help it depends on Mum's finances. you can contact them yourself.
Some types of Dementia are known to go down in steps Vascular Dementia for example,
Alzheimer's disease is usually a gradual decline.
Seeing Nita's post above, reminded me that Pain can cause a temporary worsening of symptoms. I should have thought of this before, as It turned out, some of my mum's Sundowning was caused by pain, mum was riddled with Arthritis . she was on a prescribed dose of Painkiller, which I assumed wrongly was controlling the pain. Fortunately for my mum , On the first visit of an Admiral Nurse, she took one look at mum and said she was in pain.
Once mums's pain relief was altered, her Sundowning improved greatly.
Please let us know how you get on.
Here is a link for Admiral Nurses , they are only available in the UK, sadly their are only a few of them and only a few area's are fortunate enough to have one but they do have a National helpline
We were lucky to have one here, she had more knowledge and understanding in her little finger than a lot of professionals including doctors had in their whole body.
http://www.dementiauk.org/information-support/admiral-nursing-direct/