if your Mum is bed ridden or getting very little exercise then she is likely to have a very poor transition rate in her intestinal tract. I would say it would be crucial for your mum to have a regulated diet possibly with some special additives or natural roughage/ laxitives etc.
I would contact her Doctor and ask for help with her diet, perhaps a referral to a nutritionist. You may have to pay extra for any dietary special needs at the care home but possibly just a little more care with what she is given to eat and drink. Is it possible to visit regularly to ensure she is being given fluids ? or leave bottled water or fruit drinks with her?
Why arn't all care homes required to have a proper established program to deal with residents that suffer with mental health issues.? its crazy(scuse pun). Most Dementia sufferers are unable to manage anything without prompting or help., My Father has twice stayed at a local home where he wore the same underwear continuously for 8 days and nights ! They don't seem to grasp the fact that you have to take the soiled cloathes away and issue clean ?