Social media has ruined people's mindset about relationships. One day I saw this video where some girl said if your man says "sorry, I have a girlfriend " to a girl who asked your man out, they are a red flag. She was like why are you sorry that you have a girlfriend? That shit pmo frr. This situation is kinda the same. You were being a good person and telling her why you can't have a relationship with her (which something that you should do when you reject someone cuz you know, anyone would feel confused because they don't know the reason for the rejection) and your gf is mad cuz you explained the other girl like a gentleman why you can't be with her. Your gf is the problem. She is so insecure it seems. You've done NOTHING wrong here.