Well lets just say that three months in not a long relationship at best and you are still getting to know one another. One thing I see lacking is communication. If you cannot talk about your feelings with one another, then you have no relationship. You need to sit down and lay it out there. If she feels the same about you as you do her, she will let you know and you will not have to wonder. You should also welcome her honesty with you and tell her up front that you want her honesty even if she thinks it will hurt. Friends and family aside, this is your relationship and no one else matters. I would not waste time being jealous or anxious because frankly, it leads to no where except more doubt and anxiety. Regardless of how she feels or how you feel for that matter, it needs out in the open and neither one of you should be guessing about the others intentions. I would say with a cooling off period like the one you describe after such a short time coupled with the phone conversations with the other dude, it just might be that she is not as into you as you are into her. Regardless, you need to find that out now rather than later and decide where to go from there. It might be that things are getting intense way to fast and all you need to do is slow things down a bit. But one thing you better learn right now is how to communicate with each other, or save yourself the wasted time and energy of a doomed relationship.
I think I would take a slightly more definitive approach. Telling her that you are wounded or angry when she talks to this guy will just make you look threatened or childish, in her eyes. Instead, you might take a more dignified tack. Sit down with her and say, "I think we should cancel the holiday and take a break from seeing each other. I feel you have some unfinished business with Harold, and you should probably pursue it so you can tell if it is real or not." This puts the burden back onto her to behave maturely, rather than making you look immature and jealous. If she says, "Oh, no, no, no, we're just being friendly!" you can firmly and kindly say that you feel that she and you cannot go forward successfully with this question hanging over your relationship. How she takes this will tell you a lot. She might go off and date the guy, and find out that he isn't much, and wonder what on earth she was thinking, to lose a great guy like you. (If she doesn't find this out, then what are you in a relationship for, anyway?) At the very least, this will open a whole new conversation between you. Your posture has got to be, however, the sincere desire to help her figure out what is going on with the guy in her own mind, and why she is being the way she is with him, in order for the two of you to have no doubts. Not recriminatory, not sulky, not angry. Just "helpful."
My sense is that you have made it a little too unchallenging for her to be with you. Taking this approach will regain your dignity and will also tell you what kind of intentions she really has about the relationship.
Hey man, I have a story for you. My situation is similar to yours with the exception of me being the one who ended up in the affair.
It started off innocently enough getting in contact with an ex girlfriend from highschool on a social networking site. The things started off very slow, catching up, talking about each others families, what weve done, places we've been..... innocent. (I know fully understand how volatile a situation like that is) I brought this communication up with my wife to let her know that I was indeed in contact with this woman and assured her that it was just talk.....
From there the relationship became a poison. It was insidious... I ended up wrapped up in talking about the old days and basically became emotionally involved with this woman. It only got worse from there....
With that being said, I would sit and have a heart to heart with this woman now. Dont let this go unaddressed. By my wife addressing my affair, it allowed me an opportunity to save my marraige.
I am sorry for your current situation.
You need to have a serious sit down with her. Don't accuse her or give her ultimatums. Just sit down...NO tech. Just you and her, sit down on the couch or at a table...tell her how you feel about her and ask her how she feels about you. Ask her where she sees the relationship going. Ask her about your suspicisions. Her relationship with this guy. How long have they been 'friends'? ETC...
It is possible for them to be just friends...where she loves you but he has stronger feelings for her. Please let her know that this hurts you, but DO NOT tell her to end this friendship. That would do nothing but hert your relationship with her.
If she confesses that he has strong feelings for her, let her know that you are uncomfortable with it and please respect your feelings. No late night phone calls...
If she really does have feelings for him, I am sorry...but it is better to find out now than waiting another year to find out.
Good Luck