Sexuality is a pretty personal thing - especially when you are young, and a bit insecure how you feel about it all. You should only feel a need to tell someone when YOU are ready, not because someone simply asks you. It isn't really fair for you to be asked without first wanting to tell, and it isn't really fair for someone to expect you to actually tell them something that personal. You need to explain this to your friend. She feels betrayed, and feels as if you don't trust her. Tell her that isn't it, tell her that it's personal to you, and that you told yourself you'd only tell someone when you felt completely comfortable about disclosing something like that. She should understand that, but if she doesn't, that's really her problem.
I feel confident that she will get over this - so don't let yourself get too upset about it. Usually these types of little friend-tiffs will be over as quickly as they began, but don't let that stop you from confronting the issue, and helping her understand why you was not completely comfortable with coming right out and telling her.
Never let yourself get pressured into telling someone something that personal to you. Unfortunately, sometimes friends cannot always be 100% trusted, and society is not always 100% fair to gays/lesbians/bisexuals, so you have to protect yourself from all of that. Don't feel guilty for that, feel smart.