Is it reasonable to be jealous of a guy friend? ?

I recently got back together with my ex about three weeks ago. She said that I had to earn her trust back even though I never cheated on her and I agreed. This last weekend she went on an all day trip with a "guy friend" to Las Vegas to see a concert. I wasn't jealous until I saw all the pictures on facebook of her and this guy getting real close like a couple. I feel like a fool explaining to friends that it was a platonic trip when shes got pictures of her and him close together up and none of us. What I want to know is if it's reasonable that I feel jealous and uncomfortable, especially because he drove her there and bought her dinner. I feel like this is a big strain on a relationship just starting out again. Advice?