My mom has restless leg syndrome now I think she has the beginning of dementia or sundowning. How do I know for sure?

my mom is 84 and never had any major health issues. The only thing she has is restless leg syndrome and now she has a fibrillation and I do not know if her medicine is hurting her or helping her. She doesn't sleep at night and is starting to talk while sleeping in her chair and wakes herself up. I try not to say anything cause I know she doesn't realize it. She also hears things that I do not hear. We live together. I am not sure what doctor to go to and how to approach my Mom.