I work with dementia patients daily, and yes, violent incidents do occur. However, within a care home setting, these incidents are logged as we need to see if the violence is caused by something such as a UTI (usually we do a dip asap if a person becomes very unsettled) or if it's a deterioration, in which case we need to involve other members of the MDT.
But that's in a professional setting with experienced members of staff. We need to ensure patients are well in themselves and the reason for violence investigated due to the risk of violence against themselves, other patients and visitors. It's also to ensure members of the public are safe should the person be taken outside of the care setting.
If this lady does have dementia, she needs the support of various HCPs to help her with diagnosis, and support provided. I would therefore log it with management in case nobody has picked up that this lady does in fact have dementia.
I have been assaulted regularly at work, sadly it is part of the job, but it's also quite rare (last incident I was scratched on the face and arms, left a few little scars) so when it happens, there's usually an underlying reason for it.
However, you aren't a HCP, you're working in a non-clinical environment without the support of professionals to help you should violence occur. You and other members of the public have the right to do what you're doing without being touched or assaulted in an inappropriate manner. This means it's essential the incident is logged to avoid escalation, or maybe your manager could investigate further and see if this lady needs real support.
I get other posters saying "it's only a smack on the arse, get over it" as I've nearly had my nose broken by a sudden punch to the face by a dementia patient but next time the OP might be the victim of escalation in the lady's condition. Or worse, a child (and yes, it does happen due to various complications dementia has on the brain - some patients do not recognise children as children. One of our patients had family in, another patient shoved a child of 5 over as they walked calmly down the hall).
Report it to management OP. This lady clearly needs support if she has indeed got dementia. If she genuinely hasn't then maybe try and avoid her where possible and speak to management about her inappropriate behaviour.