Her response is natural, as is your mother's. Give this time and support each other. Most elders are dehydrated and those with AD even more so. The person won't remember. The caregiver must.
Weight loss seems to be part of the course of the disease as it progresses further.
You can only give good food. The weight loss often occurs anyway. She feels she has to take them. In AD, you sometimes have to decide WHO is having the problem. In this case, partly it is your mother.