Has my girlfriends view of me changed for the worse?

I don't think there is a serious problem here. You have a very health relationship and you seem like a very nice guy for a 19 year old. Most 19 years are douche bags that just want to run around and have sex with lots of girls. You have just past the honey stage in your relationship that's all. All relationships go through this. If you have any doubts or concerns talk to her about them and let her know that you enjoy being with her and you care about her. I don't think there should be a problem unless she is losing interest. That is something that I don't understand. If I had a boyfriend like you I wouldn't be losing interest.