I have had a cat since he was a kitten about 8 years ago, about 4 years ago I adopted another male cat of similar age hoping they would play together and be friends when I started my job. They mainly ignored each other but do sleep near each other and seem to in general just exist in the same space. About 3 years ago I moved from a 2000 ft2 (186 m2) house into a 800 ft2 (74 m2) apartment and then 2 years ago I moved into a 800 ft2 (74 m2) condo. Through all of this my two cats seem to have been fine with me and each other. About a year and a half ago my girlfriend moved into my condo with her much smaller younger female (2 years). Things went OK, mostly the small female stays up on a network of shelves we built for the cats, but she can come down to eat, drink and use the litter.
We have been using Feliway since the small female moved in.
About a month ago things have gotten significantly worse, with my cat's attacking the small female when she tries to go to the litter on her own. We recently went on a trip and when we came back, there was a large number of fur piles from one of the males and the female and one of the shelves where the female hangs out had both urine and poop on it (it seems like she wasn't able go down to the litter).
Does anyone have any ideas about what might have changed? Why it got worse all of a sudden? Does anyone have any ideas on trying to make it better?