It is awfully hard to look after somebody with dementia.
I agree with CMOT wrt to getting your dad checked out - if his confusion as suddenly deteriorated he may just be brewing an infection or other; bladder infections are a common cause for this.
Having said that, some form of dementia like vascular dementia tend to deteriorate in sudden 'steps' 
- it's such a cruel disease.
Also, do go out of your way to find help - often it is not offered readily, but some help and support is out there. Speak to your SS Older People's Team: even if your mum does not wish or need carers they can advise on other organisations that can help. Consider a Day Centre for your dad to go to once or twice a week to give your mother and you a break.
Have a look on the AlzheimerUK web site - lots of information on there.
There are local groups etc you may be able to access - like I said SS and your GP's surgery should be sources of information. Does your GP have you and your mother registered as a 'carer'? This will open up some services to you, like flu vacc etc.
Please encourage your mum to accept help - plan for the worst possible scenario in terms of services, and then hope for the best. Services that you know where they are and how to access them but then never need are a Good Thing to know about - much better than to arrive at crisis point without any help available.
My MiL died last summer aged 85 - it was probably her lung cancer that killed her, but the dementia had become quite severe. We all felt that it was a blessing that she also had a physical illness IYKWIM.
My gran died 2 days ago, aged 101, having lived with dementia and very severely affected towards the end, for 15 years. It was not a pretty sight for the last 5 years or so and we all cursed her health heart that just kept on going .
Please get help in place - nobody can do this on their own for a long time.
