Boyfriend hanging out with female friend

So my boyfriend casually brought up he was hanging out with his female friend after work while we were making plans to see each other. He never mentioned it before and today was the first time. I knew of the friend before but in almost the year that we have been together he’s never hung out with her once. , am I wrong for feeling kind of upset ? I do trust him but I just find it weird how he didn’t think to mention it to me. I have also never met this friend of his either