I agree with everything Cuch said. I think what you're feeling right now is vulnerability and that can be very hard to deal with. You've opened yourself up and that can be a scary thing. However, she seems really into you. Three weeks isn't very long, and she's already told you she loves you. That's telling me that she's moving the relationship forward, not backing out.
I totally understand where you're coming from with being scared though. I think many people feel the same way, especially at the start of new relationships. Have you opened up to her about your feelings on it? I'm not sure if you feel comfortable doing so, but I think that the best way to deal with these kinds of feelings is to be open and honest about them. She may make you feel a ton better, and being open like that will definitely make you even closer. Just a thought.
Good luck! And try to enjoy the relationship and not let worry take over :)