I know! Maybe what happened yesterday could shine some light on the situation.
Yesterday when I let my kitties out to explore my backyard, another cat (an orange male) jumped in. Now all of my cats (4 females, 1 male) are fixed. But this unfixed male put them all on edge. Finally shadow attacked him and drove him out, but not before the other cats started attacking each other from the stress... it's been about 24 hours. And they seem to get along with each other okay now, all except for my little Cally. They all keep attacking her. And I woke up to Shadow (who is kind of sort of the matriarch) doing just that. Now all my little kitty can do is hide under my bed.
I think it's the stress but I just don't see why they'd pick on her, aside from her runtiness...