I can understand your concerns/guilt/worry!! I have only just come from visiting my Mom who went into a care home for "respite" this past week. She has her room permanently if she needs it. But she is HATING it!! My advice (having been down this road before - into a care home for 8 weeks, took her back out - had her back at home for 5 weeks - not coping, now back into respite care) -it is VERY early days. It can take weeks before a person starts to calm down. Also - it is, unfortunately, not uncommon for people with dementia to lose skills when moved to a new environment. When I moved my Mom here from the U.S. - she had only just retired from working full time, was living independently (albeit needing more and more help from friends/neighbors with shopping, cleaning, etc -and they were voicing concerns about her - so was in now way "normal"). Once she arrived in the UK - everything changed, from that day forward she never cooked another meal for herself, never fed or walked her dog, never did her own laundry. She stopped being able to read a clock, lost interest in telly, etc. I think I underestimated how much she was coping simply by being in an environment that was so familiar to her. She had lost the ability to re-learn things and became completely dependent on me.
Long and short - your Mum may appear to deteriorate a bit in the care home in terms of skills, this is not uncommon. In terms of feeling lonely or not being settled - this will definitely take more time to accurately judge how she is going to get on. Visit her, reassure her, take her out for walks/drives if possible. Make her room/environment as familiar as possible with photos/books/trinkets etc from home - then its just a wait and see game. Last time I took my Mom out of care -it was because she had stopped eating and I was generally concerned that she was going to die -and I didn't want her to die in the home. She is now eating again normally, and we are faced with the same problems as before - giving 24/7 care and trying to work, so we are trying a different place that is more geared towards dementia needs (rather than just a general care home). Always remember why you chose this option in the first place!