You were a rebound.
All of a sudden she'll start talking to you a bit less, find boso excuses not to see you, say she's messed up inside, things went too fast for her, she's not ready for something serious.
Next thing you'll know she'll be dating some douchebag.
If you want to keep her, your best bet is to treat her like shit and ignore her, next thing you'll know she'll be begging to suck your balls.
Girls are mostly all the same.
Most important: don't get attached, you'll just get hurt.
And about the SMS part, she's just using you to get him jealous, don't get all braggy about it.