There are a lot of reasons cats fight, some of them very silly. For example, I remember we had my long-haired cat shaved once, and the other three cats were terrified of her until it grew back! Observe your cats over the next several days to see if anything is different other than JUST their new hostility. If you're lucky, it may even fade. If not, especially if one or both of them is hurt while fighting, you may ask your vet since they have more familiarity with your cats.
Other things you can try that shouldn't harm anything are: making sure you have two litterboxes with sufficient space around each, having separate food bowls and making sure they both eat, getting one of those cat hormone air fresheners that are supposed to soothe stressed cats, or playing with your cats more. I would save more major changes to environment/diet/etc until the veterinarian recommends them.