Cats together for 3+ years and now fighting

Hey all,

I have two cats, both 4 years old (or 3 can't remember).... and they have recently started fighting, bad enough that one of them got a wound on their back that my wife saw and we thought it was something else... it is/was a scab thats now getting smaller and smaller. But now today (my wife and I were away from the house for day) I just came back to see the other cat with a large gash behind its ear. It doesn't look infected and i just cleaned it and applied some anti-bacterial cream to it. I am wondering what has brought this change of behaviour about.

These are two strictly indoor cats (both fixed,one male one female), although let on the patio (which I have recently restricted because our 'chicken wire' jail got a hole in it). Can the no going outside have really changed all of this behaviour?

They eat side by side with no hissing, but when they approach each other (recently) there is occasionally hissing, but I just saw them sitting beside each other and there was no hissing, they did 'fight' before but never with any 'damage.'

There is also a dog living in the house (but for a while 8 months) with the cats.

This recently(the fights with damage) happened after one of our cats had gone to the vet for the day to get its teeth fixed.......

Any help would be greatly appreciated as I don't want a fight t break out where a cat loses an eye or something ridiculous like that.

Yea they both seem fine, as in no abnormal amounts of sleeping etc... the one with the recent injury is now patrolling outside on the patio for the cat that lives downstairs. Those two cats have ZERO interaction as I have chicken wired my patio so that they can't get to each other.

I will take a look for the feliway. I don't know about the other cat, but I BELIEVE that it is either spayed or neutered......

I do know that as mentioned that occasionally the whole 'going to the vet seperately' can cause issues with different smells but never anything this severe

We may take the cat to the vet, there's a large amount of puss and my wife and I don't think its worth it to see if it heals itself.... :S

So took it to her the vet, apparently there was a lot of dead skin (vet said it could be a week old injury....which would be crazy, b/c we never noticed it... but thats here nor there....) Vet cut out the 'dead skin' and he suggested we could let it heal on its own (its about the size of a man's thumbnail... pretty big in my opinion) or get it sutured which would mean that she would have to go under..... WAY more pricier.... but We choose to get it fixed that way because well it just seemed like a way bigger injury and I was afraid of worse things happening....

We recently moved here 2 months ago, so the cat isn't new we are..... The problem is the balcony isn't the only place they can 'smell each other' (the other cat is an outdoor cat and goes on the shed that is right beside our balcony, which I have chicken wired in so our cats can't escape).... but there because there is a basement suite there are stairs in this old house (we live on the main floor) that go downstairs that occasionally they will sit on each side of and meow at each other....