Maybe it's just that she's coming into maturity, and even though she doesn't appear to be in heat she might still be producing a lot of hormones.
I have the mother of my kittens in a room away from the other cats and I let her in here only supervised (1. She pees on stuff when unsupervised, not sure if marking or what 2. I don't want anyone accidentally letting her outside before she's fixed). Normally she gets along with everyone just fine, but right after she came out of her last heat she kept attacking my two fixed adult cats and I ended up having to put her back in her room. She'd fluff her fur up, hiss and growl aggressively and run at them, jumping on them with claws out trying to get a fight started. She was just acting completely psychotic with them. She didn't do this with the kittens, just the adults. About a week after this she was ok again, so I think it must've had something to do with hormones from being in heat.