Adult cat suddenly scared of other cat

Question
On Sunday, we were clipping the nails of our 3-year old male neutered cat.  Part way through, he became very agitated (our female cat came in the room at the time) and he clawed us and went and hid.  He did not come out again until the next day (this has never happened before).  Since then, although he has emerged for eating and drinking, he has remained mostly secluded, mainly when the female 2 1/2 year old cat is around.  He growls at her and hides when she comes close. There have also been some extremely violent fights. Normally, they get along OK, but had their fights in the past.  However, their relationship was not atypical and traditionally male cat was the dominate one that did the chasing.  He has become withdrawn and we are concerned since we do not know what is causing this sudden behavior change.

Answer
So everything was fine until this particular Sunday, correct? This kind of reaction leads me to believe that something suddenly happended or changed to bring on this behavior. Is this the first time you have clipped his nails when she has come around? Did you happen to clip too close on accident at this same time? This is odd, but his behavior tells me for some reason, maybe a reason we don't realize, but he is blaming her for something. Or possibly she did something to him that we don't know about before you started trimming his nails.

Does he happen to growl when being picked up a certain way? Cats will often act out very randomly if they are in pain or don't feel well. How is his demeanor and appetite when she's not around?

Worst case scenario is having to start from scratch at introducing them. But for safe matters, I would recommend a vet visit to make sure his reaction isn't coming from the inside out.