@Tony123,
Tony,
If shes willing to do this when she has a boyfriend of 3 years what makes you think if you got together with her she wouldn't do the same to you? Shes not exactly exhibiting loyalty. Are you that self absorbed to think she would never do something like that to you? She doesn't have integrity to tell you to hold off your advances, since she has a boyfriend.
My theory was she was dipping her toes in to check you out. But maybe you didn't quite give her everything shes looking for OR she only wanted a quick little fling to make her feel good about herself that some random guy likes her.
You are wrapped up in her when its really not starting off very good. It's better to try and break yourself from her or else you might just continue with heart ache with her. Move on, find some another girl with more loyalty or integrity.