If she asks you why you didn't tell her sooner, you can just say you wanted to wait until you felt things were solid with your gf before telling anyone. And with this being new to you, you didn't want want to jinx anything by gushing over it before you really had a sense of how the relationship was going. How could she be mad or hurt at that? You can even say the truth: "I was a little afraid to tell you, because I wasn't sure how you'd react. I thought you might get a little protective toward me and worry, but I hope you will be happy for me."
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein