Until 2 days ago my husband who has dementia was able to dress himself and do other things. Now he can't and is more incontinent?

Sounds like something else is going on. Maybe he had a stroke? Or does he have a UTI? Time for a visit to the doc.

He could have had a stroke from his carotid artery in his neck. I'd get him to the doctor or ER immediately to see what's going on. Did you see any change in his speech or mobility during that hour when he had the pain?

Quickly have hubby checked for a urinary tract infection.... that alone can cause unusual confusion in an elder, and make him want to go to the bathroom more often. I just finished with an episode of this with my Dad [94]. Since being on anti-bionics and drinking more water so not to be dehydrated, Dad is now back to normal :)

Sounds like a stroke. Call the Dr's office or take him to the ER. This does need to be evaluated even if no specific treatment is available. they can at least prescribe treatment to they and prevent a repeat.
Does he have vascular dementia? If so mini strokes are very frequent and do contribute to a decline.