Most Helpful Girl
I agree with @yousir this day and age with hookup culture, everything is fair game until someone calls out exclusive. Have you spoken with her about it? Not sure if her age, but what's there for her to hide beyond the fact that she doesn't necessarily owe you an explanation.
Anyway, what's got you so attracted to her? Because maybe it's too fresh and new that her faults or quirks haven't had time to catch up for you to not like her. Yunno, when ugly rares it's head, it's ugly... Most people when dating or when they're fresh back into the game, they have a streak. You go through your streak and narrow down the keepers. Maybe you oughta just get your game on and check out a couple other date options. This may help you gain some perspective on this gal.
Most Helpful Guy
Here is how it works man: if you haven't asked her to be exclusive then you don't have any say on what she does with anyone else. Either nail down your relationship or just enjoy it. You can't be jealous over what is not yours to begin with.