I wished it was that easy to say someone is suffering from dimentia. It is so easy to be able to excuse recalcitrance on there must be something wrong. When people say dimentia, they are usually thinking alzheimers.
While she may be suffering from some age related memory loss and other issues related to ageing, I doubt if it is dementia that is the culpert.
While Alzheimer's has as part of its symptoms anger and combativeness in some, it is not diagnostic in itself. The main symptoms are loss of memory and disorientation.
You can read more about alzheimers at this link: http://www.alz.org/index.asp
It is just as likely, in fact more so, that she may be feeling recalcitrant and acting that way, because she is having a hard time dealing with aging and some of the things that causes.
If you get the chance, you need to see if you can get her to the doctor for a complete exam. One pretence that has worked will with some of my clients, is...lets get physical exams for a new insurance policy. Of course, a new policy may never appear. Or you can go for an exam, and then come back and tell her, the doctor said he needs to see you too..he wouldn't say why.