Why did my cats suddenly start fighting?

YES!! I have two cats and one of my cats was hospitalized for a few days and when he came home it was world war III at my house. I had to keep the hospitalized cat locked in my bedroom so my other cat wouldnt attack him. I realized that my other cat was smelling other animals on him and she became very territorial. Once the smell had worn off and we reintroduced him back they were just fine. It seems that my female cat is much more territorial than the other cat. Good luck !!!

you are very much so right, i have 3 cats all indoors and i have an escape artist my self but he is a big baby, anyways, your female cat smells the other cats and the outside and your male cat doesnt smell like him anymore so yes she is stressed, and the whole meowing thing dont give in.

This happens with my cats all the time. It sounds like a dominance issue. I know this would be hard considering you're in a studio apartment, but separating them for a while might help smooth things out. Try giving them each their own place to sleep and eat? Give them baskets at different parts of the apartment. Feed each of them in their own bowls. Also, try rewarding them for good behavior. If you see them being sweet to each other, give them treats. The next time you see the larger one attacking the smaller one, put her in in time-out, maybe in a carrier for a while. Do this every time she attacks. She should start to realize what happens when she acts badly. Not wanting to be locked up, she might stop behaving badly. If none of this works, I would ask my vet about some sort of stress and tension relieving solution. And always keep a water pistol handy, just in case.

i had 2 males cats and brought a female into the house i noticed a scratch on my oldest males face from the female cat, we had to giver her away they were ok at first then it turned bad the female kept both of my male cats away from there food and was protective off her space and me. My youngest male started to attck her more offten. I find that usually 2 males are better off togethere, female cats are very hard to keep civil they are hard to keep also because of there moods.

it sounds like your female wants to go out side also but does not have the escape artist down yet. you may want to try to get them on a leash that way they both can go outside.