A few months ago, my mom had a pacemaker installed. She was still feisty. Recently, she seems to be a different person.

I've also heard that it's quite common for people that have had heart surgery to go through a depression of sorts. Talk to her doctor.

She could possibly have a urinary tract infection. That is very common in the elderly, and that can cause someone to be a "different person". Or, there could be a problem with the Pacemaker, or she could be in pain and not be able to tell you???

Is she on new medications since operation? Look into side effects. Has she been checked for depression. Medical problems can bring depression on.