Why is our dog suddenly aggressive towards me?

We have a female yorkshire terrier (has not been fixed) that is about a 1 1/2 years old. We have had her since December 2007. She is very spoiled. She has all the toys she could want, sweaters and a coat because she only weighs 5 pounds and gets cold in the winter, and a rain coat because she doesnt like to pee in the rain. She will not eat dog food, she refuses, no matter what we buy, but we talked to our vet about that and he said some wont and the best thing for her is chicken (which she loves, and gets all the time) Anyway, she used to play all the time, and she used to like to lay down and take naps with me, and she was just so sweet. Now, when I even try to pet her, she growls and snaps at me, and she wont play....with anyone. This saddens me, I have done nothing but love her. She doesnt do it to my dad or my grandma. We took her to the vet recently because she was sick (she had hookworms, we pay a bunch for that advantage multi that is supposed to keep that away, but....it didnt) we mentioned how she had been aggressive lately, and she said "that's just how little dogs are" she wasnt like this 2 months ago.