<human>: Can you give me the gist of the text in a nutshell?

I made some new friends this year in my postgraduate course. While we supported each other with the course, we did a lot of things socially - nights out clubbing or drinking, going for lunch, supporting each other through personal problems etc. I was pretty happy because I had a great new friendship group. There were 5 of us. 

I found out a few days ago on facebook that everyone in the group except me was going to an event of one friend, Kate. Kate participated in a sports group through college and they were hosting a big ball where anyone in the group could invite others (friends, partners etc) to come along. So Kate invited everyone except me. 

No one in the group mentioned the ball to me at all. I saw them a few times at college in the last few days and it wasn't discussed in the group chat. I thought maybe I misunderstood the event and it wasn't the kind of ball I was thinking of...  

But no, I ended up seeing photos on facebook that it was just a typical ball for people to get dressed up and drink. Kate and another girl have taken all the photos of the group with all these tags alluding to them being the best of friends. 

I'm feeling pretty hurt about being excluded. I don't really know why Kate excluded me and why everyone avoided bringing it up. The only reason I can think is that perhaps it's because I'm in a long-term relationship and they are all single? 

Not sure what to do next. I don't know if it's best I take the hint and just distance myself over time, or if I should ask Kate why she didn't invite me?
<bot>: New group of friends didn't invite me to a big social event. Everyone avoided talking about it around me. Feeling hurt and confused.