sweet cat suddenly turns aggressive

Hello, my 15 year old cat Agnes has always been very sweet. I have a second cat that is 18 years old and the two have gotten along very well with one previous exception a few years ago. After the older cat had an overnight stay at the vet, Agnes greeted her back home with hissing, growling and swatting. The older cat just sat there unresponsive until the growls turned to sniffing and other curious behavior and the two were back to being friends after a day or so. Yesterday, Agnes became EXTREMELY aggressive toward the older cat. This was the situation: A lot of electrical work was being done in my apartment - a lot of noisy power tools were ripping out parts of walls and multiple groups of workmen were around. Agnes was starting to get really skittish and I was afraid she was going to run into the midst of things and get hurt, so I put her in the only room with a door - the bathroom. After a few minutes, the older cat also showed signs of getting antsy so I thought it best to put them both out of harm's way. This was a very bad idea. As soon as I stepped into the bathroom holding the older cat, Agnes screeched at her and tried to climb up my body to get at her. It scared the living daylights out of me.  I got agnes off my leg and the older cat back out the door and just put the older cat where I could see her until the workmen left. Then I let Agnes out of the bathroom and there was some hissing growling and swatting - which I expected - but after a few minutes she seemed to have calmed down and the two were OK together again. Then 30 minutes later, Agnes went on the attack again - not just hissing and growling but attacking. I want to believe that this is just a fearful reaction to the disruptions in the house and that it will go back to normal in a day or so. There was another outburst in the middle of the night and I had to separate the two cats completely. Right now, the older cat is isolated in the bathroom which I hate to leave her in, but I couldn't leave for work with Agnes still showing signs of belligerance.  I thought it best to give Agnes the space to run around the apartment in the hopes that she'll have a chance to calm down. One Footnote. There will be at least one more hour of electrical work going on in the house either today or tomorrow. My plan now, is to give her time and space and see what happens. If she doesn't calm down once the house is back to normal, I'll take her to the vet, but given her advanced age, I'm scared to death that this might be some kind of kitty dementia. Any advice?

[Tigereye1]

When animals come back from the vet they have all sorts of strange smells from the hospital.. It could be that Agnes just didn't recognize your older guy at first because he smelled like, medicine, blood, icky vet stuff, etc.. Whenever I bring my dog back from the vet my cat insists on smelling her for 10-15 minutes before she leaves her alone.. and sometimes she will hiss and swat too..

I would think that the stress from the work being done on the house is making Agnes not herself, defensive, and attack the other cat. I would keep an eye on them and wait until the house calms down. If she's still exhibiting the behavior then make a trip to the vet.

She could also be smelling/sensing something's not right with the older cat- Herd animals in the wild will attack/chase away members that are so old/sickly that they compromise the herd's safety. Cats live in colonies, so she might be doing something similar, sensing a health problem and protecting herself from it by instinct.

[zodiacqueen]

I agree with Tigereye-But, I really hope that you're not locking up the older cat since the younger cat is the one that's acting obnoxious!!  That wouldn't be fair.  I'd wait for the work to be finished in your apartment, then see if your younger cat calms down.  I'd wait 3 days until they leave, and if that doesn't work then you should take the cat to the vet.