What are signs of feline dementia? 16 year old cat had total personality change.?

He doesn't use the litter box anymore; won't eat; had a bacterial infection and is still on an antibiotic from the vet; He was previously on Prozac because he wasn't using the litter box, went back to the box, then ate litter, stopped eating his food - that's when we took him to the vet to see what was wrong; He had a 104 fever; Within 1 day after antibiotic, he was back to himself, but his personality changed; He used to be afraid of everyone and stayed upstairs; now he will not go upstairs at all; He won't play with toys; vet is stumped; physically he is better (except for not eating the past 2 days); He doesn't drink a lot of water. He was on Prozac when he had the fever - could that have effected him mentally? He even likes the dog now and he always hated her. He's not blocked - he is using my dining room as his big litter box. I was his favorite person in the world - now he doesn't care if I'm there or not there. Any ideas?