I'm in a tough situation right now and I'm not sure what to do... HELP!!! please :-)?

I have been dating the same girl now for 4 years. We are very serious and have even started talking about getting engaged in 2-3 years and getting a house or condo together etc... I just started a job that I hope turns into a successful career and so does she. Recently I stumbled across a few emails that she sent to an old friend from college that I don't know. Clearly haven't been in contact for a while and isn't one of her "good" friends that I know of and they never dated. I am going on a family trip to Vegas on March 22nd and will be gone for the weekend. Long story short, she set up a dinner date with this guy in NYC for that saturday night and hasn't told me about it. I asked her subtly if she had any plans for that weekend yet and she said no. She has never done this before and I'm not sure what it means if anything? I plan on confronting her about this before I leave if she doesn't come clean before then. Any advice or ideas as to why she might be doing this?? Thank