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."
Independent solo polyamorist seeking lover-friends willing to invest in friendship, companionship, and love, but without a need for partnership.
Never confuse commitment with exclusivity, love with ownership, nor sex with intimacy!
For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
~ Carl Sagan