Most Helpful Girl
It can be scary to think you are possibly going to be spending the rest of your life with this person or at least a good chunk of your life with this person.
Nothing wrong with being anxious. But don't let her go just because you feel anxious. Make sure that you actually want to be with her and feel rest assured if she hasn't given you a reason to leave her then things are going well.
I think it's normal to feel this way.
Most Helpful Guy
Because you're emotions are starting to cloud your judgement. It's probably a time you're getting too much into this girl and you don't know why, so you over analyze what she does and says. But you need to focus more on yourself bro, don't let your affection to this girl have control over your actions. If you feel you're getting too serious as early on, just talk to her about it.
I usually ask very casually the girls I date what they find attractive in me, or what is it that she like about us and so on... after a long period in a relationship, you have to start asking bigger things, you know, like what she wants from the relationship, or what she is expecting to gain, etc. I'm not saying you should start thinking about this now. But it's better for you to get her perspective and understand what you guys are all about (maybe she is just having fun, nothing serious).
Whatever the case may be, it's best for you to understand your place in the relationship, before you do anything stupid, IE, become needy, overly-attracted, deluded, controlling, obsessive, etc.