Gone through the part with the Senpai, where you said you'll work everything out together. Next thing you know your girlfriend is chatting and laughing with another girl, slightly ingoring you on your trip to text her and when asked about it, the reponse is "It's a secret" while smiling and slightly blushing. Reasonable to get a bit upset and jealous over, as for being childish I mean they are only 17 or so? I can still be pretty childish now, nevermind back then.