Cats fighting all the sudden

THREE YEARS ago I got a kitten and about a year later got another one. They have always gotten along. Now recently for the last two weeks roughly they have been at each other throats, literally. Fighting very aggressively and growling at each other. They can’t be around each other at all.

Nothing has changed in my house. No one is different. There’s nothing for them to fight over. They aren’t acting sickly, both eating and using the litter box just fine.

I made a vet appointment for the older one but I really don’t want to spend $400 on a vet visit for nothing.

Has anyone experienced this with their cats at all? Please help!