I'm taking it you were nervous and maybe embarrassed to say you'd never had sex with anyone else? If that's so I think you should tell her that and if she truly loves you she'll accept the fact you were just shy/nervous. I remember when I was 17 and people would ask "Have you ever had sex?" and all I'd do was turn my back or ignore them because I was too embarrassed.
Note key word: Embarrassed.
Good on you for telling the truth though. She accepted that you'd slept with 3 others and she should accept you have now only slept with her.
We're not all that bad you know