My cat isnt happy anymore

Question
I have a cat "sully" who is probably 6 or 7 years old now. We got her from a friend when she was about 7 months. Sully always had a bit of an attitude, and wasnt the most affectionate cat in the world, but she was a nice cat and would lie with you every once in a while.
Lately (approximately 4 weeks or so now), Sully has been acting very strange. She has been hiding in the basement all day unless she wants to go outside. When you go to let her out, she will hiss at you at the door until you open it. If you try and go near her, she runs away, and looks at you like you're about to hurt her. She has never been like this before.
This morning I found some blood on her neck and when I picked her up to look at it, all she did was hiss, growl, scream and try and get free. I dont know what is wrong with my cat.
There are some factors that i think may be contributing:
Last October (about a year now) we brought home a calico kitten named Seuss. Seuss and Sully never got along, but sully was NEVER like this up until a month ago. the second factor that may be causing her dysfunction, would be the fact that my brother left for university this year. Sully was always kindof his cat, slept with him at night etc. He left home in september, and shortly after he left (about a month) Sully started acting this way.
If you can please give some insight as to what may be causing her mental anguish it would be much appreciated.

Answer
Zach,

It sounds like your cat is in pain. It's possible that she has an abscess from a cat bite, or possibly something happened to her outside. Can some children have possibly hurt her? Or maybe a dog got ahold of her? Or She could have something infected or wrong inside. I would take her to the vet and have her checked out. Her behavior is extreme.

Cats do grieve for a person that is no longer there. It can last up to 6 months.

I would make sure it's not a behavioral issue before dealing with it as an emotional issue. If  it is grief or 'separation anxiety' that she is suffering from then you need to try to get her mind off it by giving her lots of kitty treats and attention. Try to interest her, if you can, with something like a pet laser light. She can chase the 'little red bug' and being energetic can help her emotional wellbeing.

You can Google "cat grief" or "cat separation anxiety" and learn more about it.

Tabbi