I think I'm in a similar situation. My gf met this guy at a party during the summer before we started dating. He's been keeping in contact with her thru AIM/phone ever since. He likes her and has been trying to convince her to go to lunch/dinner with him, even going thru their mutual friends to encourage her. He didn't know she already had a bf, so I couldn't really blame him. She agrees to lunch one day. She tells me all this and I'm disturbed by him. But I figure I should trust her and it's really not my place to tell her otherwise. So the day approaches and she mentions her "lunch date... oops, i mean day... lol" with him. I'm irked and tell her that I know it's not my place, but it still bothers me. She agrees to slip in "my boyfriend" in their conversation. Well, long story short, he continued asking to meet up with her after she says it.
I realize I'm clearly not as secure about my relationship as I thought. He, having mutual friends with her, is at an advantage and I fear he might say all the right things. I talk to her about it. In the end though, I realize I'm overreacting. She didn't have to tell me this and by putting any pressure on asking her not to see him would only work against me. If she sees him again, sure I'd be upset again. But I have to eventually learn to trust her completely.
In short, you overstep your boundaries. Apologize, tell her you were upset about it, but you'll try to deal with it. It's perfectly normal to feel jealous. But if you show her you trust her, she'll love you more for it.