My Mom is in an nursing home. She came with an uti. and received her course of antibiotics. Several days later she presented with increase confusion and even forgot who I was. I asked for another urine culture. That was refused. Nursing staff was divided if she was increased in confusion. Several days later she fell out of her chair twice during breakfast and the nurse practicioner said it was dementia. I did request a urine culture and she did get vantin antibiotic. She was less confused.
Also, if antibiotics don't do the trick, check what other meds your mom is taking -- any narcotics? Also, sometimes even simple meds, such as Claritin can cause some fog. Be persistent with your Mom's nursing team and assure them this is NOT normal. The nurse she said has dementia ... how does she know that? Has your mom been to a neurologist and officially diagnosed with dementia?