Do i have a right to be disappointed?

There has been a couple of times where my friends have done stuff without me and I don't mind. However last weekend we went out for a girls night we had a fun time, one of my friends said we should do it again next week. Next week comes along and they had a girls night but I wasn't notified. So I was a little disappointed. So one of my friends finally texts me just to say whats up we were having a cordial conversation but I felt like I needed to confront her about the situation because if I did do something to accidentally offend them I would definitely apologize. so I asked her about it but she avoided my question and started talking about something else. I felt like that was a bit childish. We've known each other for so long why can't she just be honest with me. Do I have a right to be upset?