I have recently been having a simular problem with my cats. I remembered having seen something at my vets that was advertised as good for calming cats around bonfire night. I asked about it and found out that it can be used anytime to calm cats down. It's called Feliway and comes in either a spray, or a plug in. I have the plug in and it is working really well, the cats are much calmer and are friends again. I think it might be something to do with the time of year, and as some people say, male cats being 'full of the joys of spring', if you get my meaning lol.
The plug in costs about �30 and lasts for about 4 weeks. I would definitely say that it is worth the money.