Problems with friends

I’m quite disappointed with my friends. They had asked me if I’d like to join them to bouzoukia (a kind of club where the top greek singers perform live) next week and I said that I‘ll know for sure in the beginning of February ‘cause maybe I may have to go there for another friend’s birthday. So I suggested postponing it a few days. In the beginning they said “yes” but today they told me that they’re thinking of going at the first date – probably without me.
I wouldn’t feel so disappointed – although I’d like to join them so much - if this was the only incident. I’m afraid that from the time that I have this rather serious relationship with my boyfriend I feel that my friends (all are single) keep a little distance from me and they don’t put me in their plans. I don’t like that to happen. I have made clear that there are things to do with my boyfriend and things to do with them, but I think that they feel that they have fallen from the list of my priorities and they react making plans without me. They just ask me if I want to join them or not but they don’t let me involve during the plan-making phase, like previously.