Help Understanding Cat Hissing

For the past month, my single indoor female cat Luna of about 3 years has been hissing at me at random times when I leave or come home. Also, she has been spayed and was not declawed. We've had her since she was a kitten. She was a stray that a friend of mine had found. It all started when I made the mistake of trying to introduce a stray kitten that my friend found about a month ago. At first, she seemed interested in the kitten, but as soon as we put it down, she lunged at it. Luckily, the kitten was unharmed, but I had to try hold her back using my shoes as somewhat of a shield. That night, she hissed and let out the loudest cries I have ever heard from her for about half an hour to an hour until she finally returned to normal.

I'm a college student, and it was finals time, so I was leaving for school really early and coming home really late. I believe it was two nights after the incident that I came home, and she greeted me as usual, but then all of a sudden, she hissed at me and started meowing extremely loud again. This continued throughout the week when I came home at night, and as I left for school on occasion. She would only hiss and meow at me and not anyone else in the family. She was even normal when I had friends over. Luna was always most affectionate towards me and even continued sleeping on the same bed as me after her hissing fits.

Finally when winter break came, I tried giving her more attention and spending time with her as suggested in an article I read about cat hissing. It would always make me nervous walking by her because we have narrow walkways in our house, and she would always just sit herself down right in the middle. Whenever she was like that, I would get nervous walking by and sure enough, she would start hissing. My mom is very intolerant of Luna's hissing fits, so we tried leaving her at my sister's boyfriend's house for a day since my mom was always afraid that she would attack me. She would not eat, drink, or go to the bathroom. She just hid herself in a corner and stayed there all day, so we had to take her back home.

She hissed at me less ever since then, but she still did it sometimes when I walked by. We established that my nervous reactions triggered the hissing, so I practiced walking by her calmly and then holding out my hand to let her sniff me if she started hissing. I did this a few times, and it seemed to work for a while. I figured that she was more afraid rather than angry if anything and wouldn't attack me. For about two weeks, Luna had good behavior. I played with her often and although she let out a rare hiss, she didn't let out any of those loud cries as she used to. Whenever I sensed that she was about to get into her hissing mode, I would shake her treat container to distract her and then wait until she calmed down and obeyed commands before giving her a treat.

However, about half a week after my sister went home, Luna started hissing again. My girlfriend made her a ball of yarn to play with, and the little rascal unwound the thing and tangled it up all over the house. As I was gathering it all up and stood there winding it back up, she started hissing all of a sudden. I noticed that she doesn't appear to like it very much when I just stand there, and that often triggers her hissing and shrieking. Ever since then, it's like she's gotten back into her old ways and is doing it more often.

I live in Florida, and we're supposed to have snow flurries today, so I was checking outside the window in the other room often. I closed the door to my room, leaving Luna in there so that she wouldn't get out. She was fine every time I came back, but when I actually went outside of the house to check out the weather, she hissed at me and started shrieking when I came back in and opened the door to my room. It's as if she doesn't like it when I go outside at night or in the morning because she did the same thing when I stepped out for a bit to go see a meteor shower about a month ago. I really have no idea what to do anymore. I love my cat, but at this rate, my mom is going to make me give her up.

Sorry for the super long post, but I feel like I just have to give every detail since I can't find anything similar online. In summary:

-My female cat of about 3 years has recently started hissing and shrieking at me
-Spayed, but not declawed when she was still a kitten
-The hissing started when I brought home a kitten for her to play with
-Hissing is only ever directed at me
-Hissing appears to be triggered by nervous actions, standing still in her presence, stepping outside late at night or early in the morning, and sometimes when coming home

A word of advice on yarn: could be dangerous if the cat ingests it. I have one cat who goes nuts over ribbon and I cannot have any out and about the house or she will eat it. Things like ribbon, string, yarn can wrap around the intestines and/or cause blockages.

Are there cats outside of your house? Could you be picking up their smell when you go outside? Are you spending a lot of time at a place that has other animals and bringing the smell home on you? Cats react to smell more than sight, so if you are bringing strange smells in, it could be part of or the whole problem. Also, if your cat is seeing stray cats outside, she could be misdirecting aggression at you because of seeing them.

You might want to research Feliway and try some.