My mom declined rapidly in just a few days. Could it be the Alzheimer's or a mini stroke that caused this?

It may have been a stroke or it could have been the disease affecting new areas of the brain. Alzheimer's does that. Exact symptoms can vary among people because the lesions are in different parts of the brain. Over time the lesions spread to more areas, so that functions are loss one or two at a time as the lesions become serious enough to disrupt function. It can look very similar to stroke because both are due to damage of the brain. Loss of speech and ability to do motor skills often happen during the last stage. I imagine in this late stage that the medical people are probably not as concerned why it happened as how to keep her comfortable in the time she has left. I do hope she is peaceful. I am so sorry she is going through this. Big hugs to you.