Hi Molly,
I have done quite a bit of reading up on myeloma and I would agree that confusion is unusual (although I am a patient not a professional).
To get some background, I looked at your past messages on this forum but found nothing since 8 years ago.
Has your mum been on treatment? Even if any chemo that she has been on has not been causing her trouble in the past, the effects can be cumulative, and some chemo can affect the brain.
Regards
Rabbit