My boyfriend & his ex

My boyfriend of 5 months, 2 of his friends & I went to a bar last weekend.

My boyfriend went to get us drinks & started talking to a girl. When he came over, I asked who she was & he said just a girl he knew.

I asked again who she was & his answer was the same. Something bothered me, so I asked again & he goes "that's my ex fiance".

I asked him why he tried covering up who she was & he didn't have an anwer. He knew I was upset.

About 5 minutes later, he went & talked to her for an hour and a half. The first thing they did was have a shot.

He never left for the whole hour & a half. I don't think he even looked my way.

When we left the bar, I told him how disrespectful that was. He was defensive at first, but later that night, he apologized for it, but never game me a reason.

I asked him if he was trying to prove a point to me that no one was going to tell him what to do or whatever. He said no, that wasn't it, but he could not give me a reason.

I can't stop thinking about it, because it hurt so much. Mainly because he's never done that before & quite the opposite, he's introduced me to everyone of his friends or just people he knows.

My question is, are my feelings of being hurt justified & do I have a right to expect a reason?
Thank you for all your advice.