My Chihuahua is scared of EVERYTHING?

OK, I have a teeny female chihuahua who has always been terrified of women. She didn't seem too scared of me when I got her but since I brought her home (November of 2007) everytime I have a friend over who is female she starts barking like a mad dog and goes and hides. If I try to bring her out and hold her shes sort of ok, but if the person tries to pet her or anything she hates it. It makes me so mad because I have no idea why she does this. She's so anti-social. I know for a fact that no one has ever hurt her because from the time I got her she's only been with me. I have tried to always let her sniff and smell the person that comes in contact with her to let her get comfortable with different people so that way she isn't so scared but it does no good. I have no idea. I was just wondering if someone has any idea about what her problem is. I mean I know shes a small dog and alot of small dogs are scared of everything but geez. It's like shes scared of her own shadow. I just don't know if there was something I did wrong that I need to fix or what. Thanks for any help. God Bless