This girl is really lucky to have you. Most guys, and girls for that matter, flirt with members of the opposite sex without even caring. It doesn't mean that you don't love your girlfriend. There are people who, when they are with someone, are completely incapable of looking at anyone else. Others, like myself, can still find people attractive without necessarily being attracted to them. It isn't exclusive to men, it's just that it is a natural instinct to feel attracted to more than just your mate. If you're really concerned about this, tell your girlfriend about it. She may even be able to help you. Honesty is really the best policy, and as long as you aren't thinking of these girls while you are doing sexual acts with your girlfriend or thinking about them when they aren't around, don't worry about it.
At your age, it's definitely normal. You are maybe too young to commit yourself to only one person. You need to explore a little and see what your options are. And your girlfriend should too. If you've both done some dating and then you decide to get together exclusively, then that would be great. But at your age, if you are curious and don't check out the possibilities, then you will always wonder if you've done the right thing.