Well, I don't know you, so I can't say for sure, but often feelings like this come from insecurity.
I mean, you say that you thought you'd like it, which suggests that you don't have a problem with girls having sex with other girls.
So my guesses are that it could be related to:
1. A lot of people have a problem with their girlfriend or boyfriend having previous partners because a) they didn't get to be the first; b) they worry that the other person may have pleasured their girlfriend/boyfriend better/they worry that their partner compares them to the other person
2. Related to my last point, believing the myth that girls know how to please each other better than guys do because "they know what women want". I call this a myth because all girls like different things, so just being a girl doesn't mean you'll know what another girl wants. In fact, it may have the opposite effect; a girl might assume she knows what another girl likes instead of finding out what she actually likes. I've had sex with both men and women, and their biological sex has not been an indicator of how good they are in bed.
3. Believing the myth that bisexual people are "slutty", sex-crazed, nonmonogamous or cheaters (since they won't be satisfied with being with only one sex) - I wrote an article about this topic here: link
, if you're interested.
All of these things are just speculation, of course. If you want to know what's going on with you, you need to take some time to really think about WHY it bothers you. I would also urge you to think about how this would make her a different person---she's still the same person she always has been, with all the same good qualities that make you like her. She hasn't changed at all, there is just another piece of information that you now know about her. And does the fact that she's had sex with a girl before make her a bad person?