The one that is acting aggressive behaves completely normally whenever the other one is not around. He will purr, play, sit on my lap, all the normal stuff. Both are using the litter box as normal, eating normally, and don't show signs of sickness. It has been a while since they went to the vet...
I put a calming collar on him and plugged a Feliway diffuser into the room. After a few hours I opened the door and let the one that is being aggressive out. As soon as he saw the other cat he started growling and chased him off up onto a dining room seat. I have put him back into a closed room...
So last night I was awoken at 3am to them screaming at one another, hissing, chasing, and fighting. I have never seen them like this. So I put one in the bathroom and dealed with his meowing all night.
I have seen a neighborhood cat that hangs out outside my house, but I haven't seen it around the back where the doors are. I suppose there is a chance it could have come in during this time.
I came home today to find the wind had blown open my back door. The back door had been open for who knows how long while I was at work (at most, 11 hours). When I came home I found the two cats in my room growling at one another. They have never been like this to one another. They hang out with...