Am I being overly jealous?

[OldManWinter]

So I've been dating this girl that is in the same college as me for a couple months now. We're both juniors and quite busy. I had a talk with her the other day to see where exactly we wanted our relationship to go. We both expressed that we really like each other, but decided to give it more time before making it official due to our busy schedules. We also both expressed our desire for the other person to not be dating other people, and that we were to be only seeing each other. After having this talk which I was very pleased with the outcome, I saw a post of hers on another guys wall that I couldn't ignore. Its a guy at our school that is not in our classes that I know she met only a week or so ago. He also lives in dorms that are pretty close to hers. He posted something on her wall about skiing and she responded on his wall saying that she does ski and inquired as to where he normally skis. He then responded on his wall saying "I'm going skiing tomorrow come on up!" She was working that day and didn't even see the post til late at night when she got home. She then responded saying "sorry I was busy today and didn't see this but next time hopefully I can go! You live near me right? That would be convenient for carpooling!" She did not delete the stories from her wall which showed that she was posting on his, so she apparently is not trying to hide anything.

 

I know there's no way better way to drive a girl into another guy's arms than to be whiny and jealous so I am reluctant to bring this up to her, especially since we aren't official. I would not want to ruin what we have by making her think I am some jealous, possessive guy. At the same time, I don't want to look like a fool down the road by not picking up on the warning signs, if she ends up seeing some other guy and ditches me. I've been stressed a bit about school lately so I feel like I may be letting this get to me and over reacting to what I saw, or not. I mean, we had a talk that same day and she expressed she really liked me and she definitely seemed genuine. What do you guys think, is there a legitimate concern here or am I just being overly jealous?