Sometimes it can be a little of both.
First of all, some people can't help but flirt. For example, you have a girl friend, but then this really gorgeous girl starts acting flirty with you, and you can't help but want to flirt back. Even though you're flirting, you know that you're definately not going to leave your current relationship. At an age like this, lack of commitment is very possible. Besides flirting, some girls just say that to everyone, and if you knew her before you dated her and what she was like, you should be used to it.
Secondly, I can understand that you can get jealous. But sometimes, you just don't notice how paranoid you're being.
At this age, you're going to want freedom. Being bound to one person so early in your life time is a simple waste of your teenage years. Being so protective isn't necessary. Being so suspicious will be your down fall. If you continue like this, and if this is what you're doing, I guarentee it will ruin your relationship.