When I started to be in love with this girl, my behavior changed towards other people. Suddenly I wasn't funny anymore. I laughed less. I didn't have the mood to talk to other people. I am only happy when I am with her. Is this a normal sign of falling in love? I never had a girlfriend by the way.
Love shouldn't hurt or make you sad, or less happy, it's not love because love should not stop you from interacting and being happy around other people it seems you lock off a certain part of you now you are in love, the problem is if you do this and the girl decides to leave you will be left alone because you only devoted yourself and time to this one person.
That's rather unusual. When someone's in love the 'feel good' chemicals are released in their brain regardless whether they're around their object of affection. It could be just plain infatuation- it feels like love but it's not. It could explain why you're only happy when you're with her.