I have two cats one 2 1/2 male tabby(diego) and one 14 month old female cat(RJ)both cats get on well and have since we got them. (apart from returning from vets visits when they will hiss for an hour or two) for the last three days they have been hissing growling and fighting with one another.Both cats are well. They will even sit in the same room but if left alone then a fight ensues I have no idea why this is happening but I have noticed that it is the female cat who is starting most altercations
Please let me know if you have any idea

Have both cats been neutered?

Yes wondergirl both cats have been neutered most recent being neurered around 5-6 months ago thanks

The female might be trying to take over alpha status. Can you tell if one is more dominant than the other -- seeks higher sitting places, eats first, is more pushy in general, comes first if you say, "Here, kitty, kitty"? We have four cats, including a female who desperately wanted to be alpha. The alpha male kept her in line with hisses and swats now and then. He died of congestive heart failure, and guess who took over as alpha?

Does the hissing ever go further into actual fighting?

Yes I think you could be right there wondergirl they seem to have settled down a bit today thanks