First, it's important to rule out something like a urinary tract infection (UTI) because these are notorious for making people with dementia much worse - if there is a sudden, unexplained decline then a doctor will usually check for a UTI using a dipstick first. Other infections can have the same effect too as can something like constipation. It's not known why this happens but it is a well known phenomenon. There might be no other signs of the infection, such as a cough, or smelly urine, etc and in someone without dementia they would probably go completely un-noticed.
Second, sudden declines are a defining feature of a type of dementia called Vascular Dementia. In VD you would expect to see sudden declines spaced with periods of stability. In Alzheimer's, the decline is usually gradual and steady. It can be very difficult to diagnose someone in the early stages of either of these because they both have virtually the same symptoms, however, the pattern of progress may help to tell one from another. It is also possible for someone to have both sorts at the same time (called Mixed Dementia) and also possible to have one for some time and then develop the other.
Third, Alzheimer's and Vascular are very individual diseases and no one can predict their progress or effects for one particular patient. The best that can be done is to use averages and typical effects, but there are always exceptions. For example, a typical Alzheimer's patient will have a disease from first diagnosis to death of ten years: but some people progress much faster than this, others, much more slowly. There are typical symptoms in the various stages, of which there are six, but again, some people may lose an ability at the start that is usually lost in a late stage, or retain an ability to the end that is expected to be lost early.
Fourth, most people who haven't encountered dementia do have a perception that it is about forgetting things. Obvously, this is a major feature in dementia, but there are other symptoms that are extremely common and these include personality changes, disturbed thoughts, paranoid or delusional thinking, hallucinations and so on. Not everyone gets these, but most people will get one or more of them at some time.
What you describe in the way of symptoms is, I am afraid, not unexpected in someone with dementia. They are "normal" features of someone with dementia.
However, the sudden decline is worth having investigations by a doctor because there might be a specific cause - such as the UTI - which treatment will improve. It could, however, simply be a natural progression of the disease - for your mother. Each case is unique.
Doctors are sometime sperceived as being un-coperative. It is normal for some elderly people to lose some mental ability as part of growing old - it can be difficult to distinguish between this and genuine dementia, particularly in the early stages. Whilst prompt referral is important, this has to be balanced against putting someone elderly through a set of tests and worry without enough evidence to support doing so.