This doesn't sound like Alzheimer's. Too sudden, and you haven't mentioned anything other than drowsiness, disorientation perhaps.
Rather than getting her to see "a new doctor", see if she will allow herself to be referred to a specialist who deals with something like pain management. That way the 'specialist' will be sufficiently general to be able to pick up on anything perhaps missed by her doctor, but also more able to be on the ball with more appropriate pain medications.
It sounds to me like she is being overmedicated. husband tried the Fentanyl patch and he went quite stupid on it, bombed out of his skull. My father was on morphine in his last few months and it made him hallucinate. I can take (and tolerate) morphine but when I was first put on it, the dose was too high and it made me groggy. I should have been started on a lower dose (20:20 hindsight).
If she's ever been on steroids her bones are probably like chalk. But there can be other, more sinister reasons for brittle bones, it really should be checked out thoroughly. If it turns out to be something really nasty and somewhat terminal then she STILL needs to know if only so she can get APPROPRIATE pain medications so she has quality of life and not this twilight half-world she's currently living in.
The TIAs (mini-strokes) could also cause some vagueness, but not this suddenly unless she's having them now, constantly. past ones - shouldn't do this now, if they're all in the past.
My vote is the medications being too much, too strong etc. But medically, she does need to be properly (and gently) investigated.
I don't think MRSA is terribly likely. A Baker's cyst isn't anything like a sebaceous cyst. I've got a Baker's on my knee, had it for decades (probably since I dislocated my knee at school). Sometimes it has caused pain problems, but there are other things that can be done without it needing to involve drugging her into oblivion or doing anything invasive.
I hope you can persuade her to get a specialist's opinion (pain expert, or maybe geriatrician, or rheumatologist).
Marg