It sounds to me that this is either going too fast or that you are concerned about having your time with her taken away. Some might say that you should just go out and find others to fill that time and that might be valid at stage in life, but it could also be possible that you care and love her more than she is willing to accept or invest in.
If you have boundaries about how and with whom she spends her time and what she does with that time then speak up. There is absolutely no shame in that. You say you feel like you were punched in the stomach? Tell her and ask her to investigate the feeling with you... talk about why you "think" this is... maybe it could lead somewhere.
If she is willing to talk and look at your boundaries, then you might just find that it is worth making yourself a bit vulnerable in putting your concerns out there. Relationships tend to deepen the more vulnerable we make ourselves to each other and the more we require that our partners be kind with our hearts. At least in my experience anyway.
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