Well, in a way, you aren't in the wrong, because she asked you to tell her when you do, and since you already have, she is a bit late, so you can tell her any time really. You two seem to be open about sex, or at least she wants you to be, so I suggest telling her that you've already had sex. You don't need to go into details, just tell her that you've already wanted to have sex.
She should be happy that you told her, because that is what she asked, and my guess is she will understand that you were uneasy on telling her right away.