Well this sounds pretty tough. First of all, as said in the last respones, are you sure that she wasn't like that before and you didn't notice it? It sounds like she does have a problem with you personally and sounds very insecure. I have had the same problem, but the reason was my fiancee's past. She had been heart-broken so many times in the past and got to where even if a girl looked my way she would get upset and want to beat the living hell out of her, and would accuse me of wanting her.
What was her upbringing like? how about her past relationships, if any. You need to find the insecurity and work with her on it. Let her know that there is no way to avoid looking at other people, but that you are not interested in them, that they are merely objects aound you. Also that she has your affection, not them.
I hope this didn't sound too confusing and I managed to help a little. Hope everything works out for you.