Whats wrong with my cat?

Ok, so my cat, Lucy, has been acting odd lately. She usually bites me and such, but she doesn't do it every second. Recently she's been chewing on my hand every time I go near her! My parents don't notice because they just think she's just a mean "dumpster cat" (we got her at the pound) but she's actually very affectionate towards me....usually. In fact, when I was little (even now) she protected me from ghosts that I saw or heard. Now she ignores me, and she won't play with her little mouse that she loves! She's my best friend and we grew up together, I'm hoping I don't have to lose her......please help....