Advice on a mom? Need a little venting to other brides

So my mom and I have always been open with each other. Two weeks ago she asked if I wanted to go to a Bridal Expo this weekend, it was going to be a great time to get her and my future MIL together because they haven't even met. Knowing that we were going to the expo on Saturday, I scheduled a time for my fiancé and I to meet with our pastor on Sunday before church (church starts at 4:30). Today my mom tells me she wants to go Sunday to the expo so that she can spend time with my brother and his "boyfriend" (they've been fwb for a couple months) on Saturday. I'm pretty upset and I'm wondering, am I being bridezilla or is this understandable to be upset about?