Why is my cat acting like this?

My 8 month old cat (female) acted the same way when I brought a 3 month old kitten (female) into my apartment. Give her a few days to adjust, maybe even a couple of weeks. Most cats will tolerate each other after a while. They are even more territorial than dogs, and bobcat is freaking because there's a new cat in her territory. Once they get used to each other they will use the same litter box and food dish, though probably not at the same time for quite a while.