How do I know if my girlfriend has genuine love towards me or not?

Well, my girlfriend and I have been dating for a little more than two months and lately I will casually say "I love you" but she will just say "you too" or not reply at all. At first I was concerned that I was making her feel uncomfortable but I'm not sure because she has said "I love you too" in reply in a far more cheerful manner. And please don't give me the "your too young to be In love" crap, because I am well aware I love her with all of my Well-being. Also she doesn't treat me like my past girlfriends (say I love you, complement me, and is just nice in general). Could it be because of age? Or has she lied about loving me? Or is it she is confused with these emotions? because according to her mom I'm her first "real" boyfriend. Please help D: