Mother with dementia has stopped going to the toilet properly, when she should. What to do?

Over the last week or so, my mother (about 74) finds it hard to go to the toilet, in order to use it. When I stayed with her [and father (76)] in my hometown during the entire past August , she would head for the restroom, or I would take her by hand there, at least three times a day. With me, she was really cooperative, and we were glad about that.
Now they're back with the middle brother's family, and l help around on weekends. Here, where the restroom/toilet is easier for mom to notice and access, she appears to have lost all sense of orientation, and tends to seek other corners to urinate and/or defecate, which she did the other night - in her bedroom, though she was led into the toilet earlier by both father and sister-in-law, so as to prevent the very accident!
It's crystal clear mom isn't doing this on purpose, but I feel so surprised at this abrupt change in her. What is happening with mom, and what can we really do?
Thanks.. 

P.S. In a couple of weeks, my middle brother's family intends to move into their own place, which is one floor above where they're living. My oldest brother's highly indifferent family lives in the same building on the third floor. So, father and mother will be staying on the first floor. This upcoming occurence has also made father feel worse, given that mother isn't cooperative [enough] with him..