should I be mad about my guy going out without me?

My boyfriend of a month and a half has introduced me to all of his friends pretty much now. The one's I met love me. The entire night they were so sweet and we all had fun. I'm very easy going and can get along with anyone. But he has this one friend Nick (who I have met one on one) when he goes out with him he doesn't invite me. First, it was a couple weeks ago with a Canada trip. Someone else's GF came though that night. Now I just asked my BF if he could come over Saturday and he said he has this girls birthday bonfire to go to with that same friend Nick.

Should I be a little concerned about this? I told him not being invited to Canada bummed me out and it's like he completely ignored what I said. Why would he not want me to come when all his friends kept saying how awesome I was?