Should I be upset

My fiance's birthday is december 9th. My sister is having a girls only baby shower on december 7th. My fiance is upset that I am going to her shower 2 days before his birthday.

He posted on facebook about going out for his bday weekend and a girl who actually told him she used to have feelings for him, wants to get together with him and he's doing it!!! I'm pretty pissed off. This dude wont even let me talk to a guy, let alone hang out with one. Especially someone who used to have feelings for me.

I'm pretty pissed. He says I shouldn't be mad, but idk I disagree. I just know if this were the other way around he would throw a fit and think I'm cheating on him.

Thoughts?