In addition, I want to give you some tips about relationships with American girls.
You should be aware that most American girls date several guys at the same time. Most Americans meet their dating partners in this way. You could be one of the guys she is dating now. But that is not a problem in the view of American culture.
Most important thing is you should ask her to meet you exclusively. That means you and she would be in a relationship. Being in a relationship means you and she are dating with marriage in mind or committing to date each other exclusively. Before you ask her about it and she answers yes, what you are doing with her is just a casual dating.
I am not sure whether you have a sex with her already. If you got her to make out with you, I recommend you to ask her to meet exclusively. Also, you should ask her what she really wants in the relationship. Some American girls do not want to marry and just want dating partners or friends of benefit.
The most common problem of American girls, especially white girls, they are not faithful to relationships. Some of them enjoy multiple relationships or open relationships even after they are married. Some white girls keep contacting their male friends or their ex by text or Facebook. And they keep friendship or relationship even after they are married.
Please check whether she sends texts or Facebook messages often when you are dating her. If so, she might be just a common American girls. If you are added to her Facebook, you can check her Facebook friends list. If she restricted to see her all friends list, that means her relationship is complex. Also, if she does not get back to your message quickly or decline to answer your call without any excuses, that means she is dating someone else at that time. If she cancel appointments or say unreasonable excuses, that means she does not put her priority on you.
It is most important to check whether your date is faithful to a relationship. Actually, it is hard to find American girls who are faithful to relationships, especially for white girls.